The subjunctive mood is used to express actions or ideas which are subjective or otherwise uncertain: will/wanting, emotion, doubt, possibility, necessity, judgment. It is nearly always found in dependent clauses introduced by que or qui, and the subjects of the dependent and main clauses are usually different.
Je veux que tu le fasses.
I want you to do it.
Il faut que j'aie de la confiance.
it is necessary that I have confidence.
The subjunctive can seem overwhelming, but the thing to remember is the subjunctive = subjectivity, unreality. That should help you figure it out at least 90% of the time.