The first thing to do is to go through the drive contents and delete anything which you don't need.
If there are backed up files on the drive you might transfer them to an external location - either an external hard drive, or to USB sticks, or to DVDs /CDs
Drive management involves regular maintenance tasks, and this involves scanning the drive for redundant files - CCleaner is an excellent free utility that will clean up the drive for you. In general terms you can tell it to delete everything it finds.
Run the Windows de-fragmenting tool as well. This will organise software file structures. Repeatedly opening and closing software applications can cause files to become scattered all over the drive space. Defragmenting a drive reorganises the file tables.