A contract is legally binding, meaning you are required by law to uphold your part of the agreement. If you don't, you could be sued or face other severe consequences (and remember, now that you are 18, you can be sued in your own name, and will be responsible for all consequences that accompany this).
Remember a contract can be in writing or it can be an oral agreement. The only difference becomes one of proof in court - but if you, as a now-competent adult - agree to do something and the other person relies upon it, you may be held responsible if you fail to fulfill the agreement.
Contracts will likely become a frequent part of your adult life, and you will often enter into one without realizing its significance as a "contract":
- When you join a gym, rent an apartment, or buy a cell phone, you will sign a contract.
- Contracts do not all look the same: you often enter into contracts online by clicking a box that states, "I have read and agree to the terms & conditions."
Don't sign anything unless you understand, and have read, all components of the agreement.
Don't alter any provisions of the contract without consulting the other party and check to make sure that the other party has not altered any terms. If they make alterations and you sign, you will be deemed to have agreed to those alterations.
Never sign a contract with blank spaces.
Keep a complete and signed copy of any contract you sign (in case you need it for future reference).