Who to battle when first able to access gyms?

Solution 1:

Unfortunately due to the design of the game, people who start late are going to be at a massive disadvantage when taking gyms as the higher level players would be constantly taking them with high CP Pokemon.

It's likely going to be impossible for you to get 10 gyms until you get to a much higher level where you have access to higher CP pokemon. Due to the fact that the xp required to level increases by an insane amount at around level 20 and beyond, you could catch up somewhat. At around 20 - 25 you should be strong enough to defeat a level 2 or 3 gym easily by yourself.

Some things you can do to level the playing field at a low level is to consider the types of pokemon. You will want a Pokemon that takes reduced damage and deals extra damage to the ones in the gym. You can beat pokemon up to double your CP in this way.

Another thing to consider is dodging. Dodge when the yellow sparks appear and you can reduce or completely negate the damage dealt by the enemy pokemon. This makes it easier to defeat higher pokemon.

The final piece of advice is to team up with a friend or two. I've found that all gyms are pretty easy when it's 2v1 (total of like 12 v 2 or 3) as you are effectively doubling your attack (it also doubles your HP and the enemy's attack, but these two cancel out essentially, or you can dodge to negate the attack as explained previously).