Becoming a confidential informant for the police is a high-risk and high-reward option. At best, you avoid being charged with a crime by working for the police in making cases against bigger targets. At worst, the police string you along until you are no longer valuable to them, then they charge you with the original crime.
Only a prosecutor can make deals with you. The police cannot promise you anything--even though they will lie to you and manipulate you into thinking the opposite. For example, the police can tell you to do an undercover buy with one dealer, and then demand ten more undercover buys, and then ten more, and then pop you on the original charges. Of course, you also expose yourself to high risk by snitching.
The best way to protect yourself while becoming a confidential informant is to be represented by a lawyer who is negotiating directly with a prosecutor on your behalf. Only a lawyer will be able to adequately protect you by getting your agreement with the State in writing--and with someone who actually has the power to bargain on behalf of the State.
If you're in this situation, call me right away to talk through your options.