Details
A must have for NFPs
Setup
What ever you do, get the trial version already set up. Setting it up is a pain. Get the pre-configured trial here http://www.salesforcefoundation.org/tryitnow
If you have to set it up yourself follow the instructions here https://help.salesforce.com/HTViewSolution?id=000005149
Then go through the post install steps starting here https://powerofus.force.com/articles/Resource/NPSP-Post-Install-Checklist
Using NPSP
See Using the Non Profit Starter Pack for more details.