Here are some tips from me:
Use your Nerve to increase your Crime level (if your Nerve runs out, you can use cannabis to gain 8–12 Nerve).
Use your Energy for the Gym (to increase your battle stats).
Or you can choose to spend your Energy attacking and killing higher-level players (but your battle stats must be high).
Every time you finish attacking someone, choose the “Leave” option instead of mugging or hospitalizing them. Leaving gives you more EXP.