if you live in Pennsylvania, March 24th is the deadline for registering to vote.
If you are registered already as an Independent, know that YOU CANNOT VOTE IN THE PRIMARY. Pennsylvania has what is called a "closed" primary. In order to vote in the primary, you must be registered with one party or the other. In other words, you must be registered as a republican (and then you only vote in the republican primary) OR a democrat and then you only may vote in the democratic primary.
For the forms you need to register or switch affiliation from Independent, go here.