What do I need to do to get my computer ready to sell?

For the most part, a simple format and reinstall should do the trick, which should be simple to do if you have a bootable operating system disc/usb drive.

If you really want to clean your records off of your hard drive, you could use something like Active Killdisk, which overwrites data on the disk via multiple passes so no data could be recovered.

If you don't already have an external hard drive you can buy a drive for around 60 or 70 dollars. This 1TB drive should be more than enough for an old desktop computer considering they had at max 500gb on most. Basically you should drag and drop all of your important files such as pictures and movies to the drive as well as any documents you might have that are important. If you want to guarantee that no one will be able to recover any data on the hard drive I would suggest to take the hard drive out and run a magnet over the disk a couple of times to scramble and corrupt any data that might be left after you transfer the files to the external drive. (You can then simply buy a cheap 500gb or 300gb internal drive and put that in the old computer to sell). Before you sell it it helps to clean out any dust that might be inside of the computer and outside to make it look clean and to ensure it runs effectively. MiniTool Drive Wipe is a great free program to erase all of your hard drive data too if you don't want to keep the old hard drive. It is free and you can download it from their website. The program makes 35 passes with the hard drive and once it is run nothing will be able to be recovered. I am not sure if it deletes the Operating System too, but I would think it does. If you have the CD for the Operating System you will just need to load it back on the hard drive after it is erased. Literally nothing will be left on the drive after you use most programs to erase it unless you find one that does not delete the Operating System.