Hi & welcome.
Basically, you don't "run out of" memory. What you've got is what you've got. Using lots of it is fine - that's what it's there for. Data gets flushed out of your memory whenever you shut your computer down. Simple explanation, but it'll do.
Your hard drive
does get filled up over time. That's what holds all your 'stuff'. You can always pick up another hard drive and simply add it to the one you've got, as additional storage. More space to hold your 'stuff'. First, though, why not clean up the drive you've got. Start with a nice program like
CrapCleaner <---click here. After you clean things up, run Disk Defrag. Then reboot and see how much hard drive space you've regained.