All the software on that computer belongs to the Apple ID of the previous owner. You cannot keep it. You will have to repurchase the software under your own Apple ID. The exception would be any software that came on a DVD or downloaded from another source than the Mac App Store or the iTunes App Store. This applies to the operating system, as well. That means you must repartition the hard drive for a complete wipe and start by reinstalling Snow Leopard 10.6.8 or the original OS X pre-installed on the computer when it was new.
There's little more we can help with at this time without knowing more. Did the computer come originally with Snow Leopard or earlier in which case there should be software restore DVDs for the computer. If it came with Lion or later, then re-setting the computer to its original state will require an Internet Recovery.
You will need to get an Apple ID and password to use with the computer. This comes from Apple once you get the computer re-setup using iTunes.