While I find the current debate nice to read, apparently for a player like me at level 15, I'm not exactly sure when can I start getting Gyms? Long story short, I'm getting body bagged.
Since you're at a fairly low level, catch pretty much everything and keep as many candies for pokemon, especially (but not exclusively) those with 12 or 25 candy evolutions, whose evolutionary lines you have filled out (although you should probably avoid wasting the candy of valuable species; anything in this list is useful to some extent:
https://pokemongo.gamepress.gg/gym-attackers-tier-list). Once you get around 70+ pokemon that you can evolve, use a Lucky Egg and keep evolving stuff in the 30 minutes until it runs out.
The higher your level, the easier doing anything gym-related will be. Although I'm not really sure how taking down Blissey works at low trainer levels (without incredible dodging? idk), pretty much everything else can be drudged through. After you get a couple solid Vaporeon, consider also getting a few good Electric types (Jolteon is the strongest, but things like Raichu and Lanturn will work) just to go up against Gyarados. Attacking Pokemon like Rhydon and Gyarados that have prominent quad weaks might allow you to raise the exp of low-level gyms on your team enough to raise their level and put one of your own mons in.
Putting in your own stuff means more coins, which means more incubators (you probably shouldn't purchase the lucky eggs with coins, as you get a fairly stable supply from level-ups and they don't really give you shots at better Pokemon). More incubators means more shots at high IV and high value Pokemon. Be sure to only use your limited use incubators on 10k eggs. Similarly, try to save your 10k eggs until you get a limited-use incubator to put them in.
Make sure to do at least one stop a day + catch at least one mon a day. This isn't too important, but it it's free exp and makes keeping your stardust high (you'll need it, eventually) easier in the future.
Keep an eye out on the silph atlas (here:
https://thesilphroad.com/atlas ) for useful nests in your area.
Oh, and make sure to always curve every ball. It's a lot easier than the straight throw once you get used to it and also ups catch rate.