My main piece of advice would be to catch as many Pidgey/Caterpie/Weedle as you can, and then evolve all of them at once while using a Lucky Egg - you can get a tonne of exp doing that. I think the highest number of evolutions you can use during one Lucky Egg is like 50 or something, and at 1000exp per evolution, that's a lot of exp.
If you manage to take over a gym, I find it's best to take the bonuses (from the shop) as soon as possible, because it's unlikely you'll get another Pokémon into a gym before you lose the first gym! Unless you manage to get into a mega gym that's like level 10 and has a ridiculously high CP Pokémon in it.
Don't waste stardust/candy on low CP Pokémon, even if they have good IVs, unless you happen to live in a rare Pokémon spawn and have a buttload of candy (I live next to a Magmar spawn which I think just swapped to a Pinsir spawn). Even then I'd still just wait until you find a good one with an already high CP. Powering up your starter is usually a waste of time and resources, but obviously if you really want to there's nothing stopping you.
Also join Team Instinct woo