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 or here https://powerofus.force.com/articles/Resource/Install-NPSP
Then go through the post install steps starting here https://powerofus.force.com/articles/Resource/NPSP-Post-Install-Checklist
Using NPSP
See Using the Nonprofit Success Pack for more details.
Upgrading
Many NFPs are probably still using the NPSP Version 2. See which version you are using by following these steps https://powerofus.force.com/articles/Resource/Which-Version-of-NPSP-Am-I-Using
Upgrade using these steps https://powerofus.force.com/articles/Resource/Upgrade-to-NPSP3-from-a-Previous-Version-of-NPSP