Step 1: Backup Drive
Buy an external USB 3.0 drive you can store backups on. Preferably 1TB minimum.
Back up with Time Machine, and then eject and unplug the drive.
Go ham and delete applications you don't need/use, old files. If you ever need a file you're missing, just grab it off your Time Machine backup.
Step 2: Cloud Storage
Find documents you no longer are currently using, but need to save.
Drag and Drop them into your iCloud Drive in Organized Folders. Spotlight will help you find them.
Step 3: Uninstall old apps
Find applications you don't need and get rid of them. Open Applications, and move old apps to trash.
Step 4: CCleaner
Download and install Piriform's Ccleaner. You may have to allow the 3rd party app to run. If it doesn't open, then go to System Preferences > Security and click 'Allow anyway'.
Once you open the program, just uncheck items you don't want to lose (such as cookies for Safari/Chrome/FF). You will have to log in to your websites again, just so you know.