Multiple databases or tables with prefix?
It's not simply a question of plugging values into an equation and, hey presto, you have an answer (as @dbdemon also points out - no standard solution).  

You need to put considerable thought into this - it could be a choice which affects you for a considerable time.

A few things you should do before you start coding:


Gather detailled requirements (as kindly suggested by @Peter) 
Check out the threads and a few of my answers on multi-tenancy here. Also, as suggested by @Colin'tHart, do a search on this site for other posts related to multi-tenancy.
You could also download a few Open Source apps - use your search engine of choice - duckduckgo got me a few links.
Take a look at databaseanswers and their schemas - something there might be of interest/help.
Do a bit of prototyping - see what "fits" with your company/clients/stakeholders requirements and/or your company's skillset in the coding/database areas.