...
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 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