A must have for NFPs
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
See Using the Nonprofit Success Pack for more details.
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
The upgrade process is super easy, I was pleasantly surprised, but then you have to deal with your households. Go here https://powerofus.force.com/articles/Resource/NPSP-Convert-an-Existing-Account-Model-to-the-Household-Account-Model and if you are not freaked out by checking ALL those boxes before you start, then stop what you are doing and understand it first - this is a big deal.
I would then rename or remove the Household object, and remove all the Household reports from the Report Types list.