If you're just asking to how to register for the GPL, you will be able to register at thegpl.com
once they announce dates and locations and such. You'll also need individual player IDs so your eligibility can be tracked.
If you're asking how to get into speedball more generally, I suggest finding your local field and playing there as often as possible and looking for other fields in your region and traveling there to play against other teams occasionally. Where are you located?