and welcome to BC!
and The weatherman
already mentioned you can defrag your computer using the Windows Defragger. However, I personally don't really like the Windows built in one because it's not really that good at defraging some files. Overall it does an okay job and get's most files defragged but I didn't find gaining much system performances on it and when I used a different defragger it helped defrag alot more files that windows built-in one couldn't. It's still not a bad defragger but you probably can download a free and better one.
With that said, you can download a defragger tool and use that. The ones I use to defrag my system once a month or twice a month is Auslogics Disk Defrag
It takes less than 5 minutes to defrag your system. If your have more than one drive meaning if it's partitioned, like mine then you will need to do them one at a time. I really like it, and there's another tool that is good at defragging specific files and folders on your system which also does a good job, but it takes longer. It's a very small file and you can even carry it on a flash-drive, it's called Defraggler by Piriform Defraggler Home Page
Also defragging your system doesn't alway help increase system performance, sometimes there are some other things you need to take care of as well. These include: disabling startups or services, temporarily internet files etc... Some helpful tips on slow computer can be found over here
Go to Start>>Run>> Type in Cleanmgr
This tool in windows can help you compress old files, delete temporarily internet files and many more.
Hope that helps.