New player needs some help.

How to choose my targets ?

I'm regularly losing against afk people who only have some body armour who are within my level. Level 8 atm, maybe my stats are to low? Should I aim to attack people who are way lower? Armour seems very expensive expecting for the very basic one.

What goes faster for leveling? Attacking people or using my energy for the gym?

Attacking is the fastest way of gaining levels. But leveling won't help you so much, it could give you a false sense of how strong you are relative to people. It's very easy to level up quickly and leave your battle stats far behind the average.

Just keep training instead of attacking people. Have you looked at Duke missions? those are almost always really easy targets.

The goal right now is to get to level 15. I probably could focus on train once I'm level 15 of would that make me an easy pray?

I got to level 15 not to long ago and I naturally leveled up by gyming and crimes for 2 lvls and I can tell you that if you reach 15 by going to fast and you travel you will be a super easy target and it will just make traveling more difficult.