Conspiracy is generally an agreement between two or more people to commit a crime or some other illegal act. For example, several people agree to sell drugs. Conspiracy requires proof of at least two "over acts" committed in furtherance of the conspiracy. Agreeing to sell drugs is not enough - the conspirators must then do something else, like go buy the drugs to sell, contact some buyers, meet them at McDonalds to make the sale, etc. But it is the agreement itself that is the crime.
It is not as easy to prove as it might seem. Usually one of the "co-conspirators" will testify against the others, unless, as is frequently the situation in federal drug cases, someone else actually overheard the conspirators making their arrangements. Conspiracy is not often charged all by itself; it is usually one of the several charges filed against the defendant(s), and it is the "catch all" type of charge. For example, if the prosecution cannot prove the underlying attempted drug sale for some reason (no drugs found at the scene), they rely on the conspiracy count(s) to at least get some conviction. But this usually involves cutting a deal with someone to get their testimony to convict the other(s).
So be careful of your friends! Consult with a qualified criminal defense lawyer - talk to nobody but him/her about your case or situation.
Conspiracy is an agreement between two or more people to do a criminal act. They must also do something "in furtherance" of the conspiracy. That means they have to physically do something. Two people just agreeing to do a crime isn't enough unless they act on it.
In a conspiracy, all participants are equally responsible for the underlying crime and any consequences of it, even by other members of the conspiracy they haven't met.
The DA would have to prove the agreement to do the underlying criminal act and some "overt act" taken towards completing the act.
As indicated above, conspiracy requires 2 or more people agreeing to commiting a criminal act. There also must be some overt act take towards the conspiracy. There are many complicated issues that can arise, that either show that someone is not part of a particular act, that they do not know about or that they are responsible, because it was reasonably forseable that an act would occur in the future.
An experienced criminal defense attorney could evaluate the facts surrounding the case you have and give a more detailed and specific answer.