I have a couple questions:
What should i be saving my coins for?
Also, i captured a 15/14/15 Rattata, i evolved it into Raticate and it had the moves Bite and Hyper Beam. Is this a good Raticate or are there better ones?
Raticale is fine like that, but it will never be worth using dust on. It simply isn't a strong enough pokemon to use limited resources to power up.
The stamina is majorly broken and the effect speed has on combat performance is also broken, but to a lesser extent.
Currently, Stamina is 2*Base HP, which really hurts low HP pokemon because it doesn't take into account the 1 HP per level pokemon in the games normally get. The formula for stamina should really be around 65+Base HP. This would put Vaporeon's HP at 50% more than Jolteon or Flareon...far more accurate.
As a result of this, speed, and the lowered effect of super effective/not very effective and STAB, only a few pokemon with the right IVs and moveset are worth leveling up with your stardust. One thing to note is, generally, fighting pokemon, rock pokemon, and electric pokemon are pretty terrible due to the moves they can have and this creates a skewed metagame along with the unbalanced stat interpretations.
I will give you my perspective as someone on level 29 with a pokedex of 130.
There are good offensive pokemon and good defensive pokemon. I will start with some examples and my opinions on what to do with defensive pokemon.
Defense Pokemon:
In my opinion, defense pokemon are almost never worth powering up if you won't ever use them offensively. The reason being several hundred CP will not make your pokemon significantly more effective against anyone who is competent at dodging. Defensive pokemon worth keeping simply to drop into gyms. Some defensive pokemon of note are:
Snorlax - Zen Headbutt/Any Charge Move - Annoying defender, almost guaranteed to at least dent an offensive pokemon. No real weaknesses, huge benefactor of stat interpretation. If it has EQ, Exeggutor will have his number relatively easily.
Exeggutor - Confusion/Psychic - Exeggutor is relatively obnoxious with double psychic abilties because it will hit virtually any good pokemon for neutral damage. I await the day Houndoom arrives to makes his life miserable.
Dragonite - Any abilities - To be honest, for his CP, Dragonite really isn't that bad to deal with. Of course, I say this as a person who has a frost breath/blizzard cloyster who dumps on him all the time. He resists all the common types and has boatloads of CP, but he isn't that hard to beat.
Lapras - Ice Shard/Charge Move - Lapras is also relatively obnoxious to deal with. Like Snorlax, he greatly benefits from high HP and low speed in PoGo.
Poliwrath - Bubble/Hydro Pump - Bubble is broken due to its speed and defensive double cast mechanics, but otherwise, Poliwrath isn't that special. Just a good defender.
Slowbro - Confusion/Psychic or Pulse - Again, broken record, good defender. In particular, with psychic, he can handle the grass/poison pokemon who like to counter waters.
Vaporeon - Water Gun/Any - Broken amounts of HP. Not actually a great defender, but they will be almost everywhere due to ease of access.
Victreebel/Vileplume/Venusaur - Razor Leaf/Any - Grass pokemon will be thrown in to disturb your offensive water rhythm. They can be handled by fire relatively easily.
Arcanine - Fire Fang/Flamethrower or Fire Blast - One of two fire pokemon you are likely to see often along with Flareon. Mostly fodder if you ask me because Vaporeon is so dominant on offense.
Flareon - Ember/Any - Like Arcanine, truly fodder, but a rhythm breaker.
Offense Pokemon:
I think offense pokemon are the best to power up for a variety of reasons. They are the most important for actually taking gyms and they are the part you control the most. They are the ones who do the serious heavy lifting. After you hit somewhere in the low 20s, it will be worth keeping track of your best IV, quality moveset offensive pokemon for taking on gyms. Primarily, the ones who are best for dealing with the above threats.
Vaporeon - Water Gun/Hydro Pump - Vaporeon is seriously the most ridiculous offensive pokemon right now. Water Gun is excellently spammable and many pokemon fall in that HP range where hydro pump just finishes them off and prevents them from getting the next charge attack off. She will beat just about anything head to head with little difficulty except possibly Exeggutor, double psychic Slowbro, Dragonite, and the grass pokemon. She dominates most of the other water pokemon because of offensive advantage, she bests Snorlax for the same reason. She rolls through basically everything you need her to. I have 7 90% plus Vaporeons with this moveset because I've caught over 500 eevee. I keep 3 at max standing.
Exeggutor - Zen Headbutt/Solar Beam - This is the other MVP of offense. He hammers the grass pokemon, the water pokemon, and will beat Snorlax one on one. If you see a fire pokemon, you simply switch to Vaporeon and dismantle them. These two form 2/3rds of a FWG core that is great for offense. There are other grass pokemon who can provide some offensive options. The ones who defend above are viable options. Venusaur is superior on offense if it has Vine Whip as a pure grass pokemon. Zen headbutt/Psychic has applications in particular for neutral matchups if you just want a psychic pokemon who wont fall over when the wind blows.
Venusaur - Vine Whip/Solar Beam - Very strong against water pokemon. Vine whip is excellent on offense. If you hatch a very good Bulbasaur and eventually get enough candies, he can be your go to if you can't manage to get Exeggutor to agree with you. He can perform many of the same functions, though he has less overall coverage outside of pummeling water pokemon.
Arcanine - Fire Fang/Fire Blast - Arcanine will have the best matchup against Lapras and grass type pokemon. Flareon is a slightly inferior version of Arcanine, but has the same general effect if Growlithe is rare in your area. He can handle them with ease, though if you are feeling lazy and don't dodge, Exeggutor can hammer you with Confusion/Psychic. The rest fall quickly.
Lapras - Either/Ice Beam or Blizzard - Lapras dominates Dragonite, can actually handle Exeggutor easily as well. He can also win stall wars with the waters due to resisting them just as easily. While he is better defensively, he is just as good offensively for handling annoying pokemon.
Wigglytuff - Pound/Hyper Beam or Play Rough - Wiggly is to offense what Snorlax is to defense. She has a neutral matchup and extremely strong moves for offense in both slots. She massacres Dragonites running Dragon Breath, and the Pound/Hyper Beam combo in particular will easily take out any pokemon you need her to if you have a weakness in your lineup. The reason Wigglytuff needs to be mentioned is the ease at which she will find candy as your buddy. Jigglypuff candy is 100% the most useful candy per kilometer you can get for battling and it, frankly, is not even close. She has a ton of HP and does a ton of damage.
Substitutes to those above: Victreebel/Vileplume with Razor Leaf and SolarBeam/LeafBlade/PetalBlizzard for Venusaur and Flareon. Charizard serves as a sub except against Lapras. Since offensive ice is relatively niche and aimed at stopping Dragonites, both Dewgong and Cloyster can fulfill the same role with the proper moveset.
Outside of these, I am not sure there is a pokemon I would keep powered up. If you want some numbers, here is what I have powered up most of the time:
3 Vaporeons with Water Gun and Hydro Pump
3 Exeggutors with Zen Headbutt and SolarBeam(x2) or Psychic(x1)
1 Venusaur with Vine Whip and Solar Beam
1 Victreebel with Razor Leaf and Leaf Blade
1 Arcanine with Fire Fang and Fire Blast
1 Flareon with Ember and Fire Blast
1 Cloyster with Frost Breath and Blizzard
1 Wigglytuff with Pound and Hyper Beam
1 Jolteon with 100% IVs, Thundershock and Thunder, who I don't want to feel bad for sucking.
That is where we are currently in this meta. There is really no point to have much of anything else. They need to fix the stat interpretations, move balancing, and probably remove STAB altogether in a game where you don't have enough control over your moves.
This post became way longer than I intended.