There are many different ways to make gain experience in order to increase to higher levels within the game and to improve your stats. You gain a new level when your Exp reaches 100%. When you gain a level your Exp returns back to 0%, but you gain an increase in your total Energy, Will and Health. There are 5 types of player stats used on Criminal Fire;

Strength determines how much damage you do in battle.
Guard reduces the amount of damage done to you when you are hit.
Agility is used to determine your hit rate in battle.
Labour and IQ are used to determine what jobs you are able to do.

Improving you player stats

Gym: To use the gym, type in the number of times you want to train, select the stat to train and click ok. The next screen will tell you how much of that stat you gained, and what your total in that stat is.

School: School offers courses that will raise your stats after a certain period of time of learning.

Your Job: A job will provide you with money at 5:00PM everyday, as well as raising your job stats everyday. Some jobs have requirements before you can do them, so make sure to keep an eye out for that.

Gaining experience to gain levels

Crimes: Go to the crime screen and select the crime you want to attempt. Remember that trying a crime that is too hard may land you in jail, and lose the experience you've worked so hard to get. Increasing your Will by gaining levels and by purchasing more expensive houses will make crimes easier to pull off.

Attacking: Attacking will gain you experience when you win, but you lose experience if you lose. The amount of experience depends on the comparative strength of your enemy, if they are much weaker, you won't get much experience. On the other hand if they are too strong you, it will end in a small experience loss along with a trip to hospital for you while your victim can gloat over their victory.