Firstly, how much hard drive space is used up and how much do you have in total?
Second, if you have money to spend, an external hard drive will serve you very well in keeping keeping your primary hard drive more clean since you can store tons of files such as videos and images and everything not necessary to be stored on the main hard drive like such as software and games etc.. on it.
Third,
WinDirStat is an excellent tool for getting an quick and accurate overlook on what is taking space on your hard drive; it scans all your files and folders and with a folder based structure gives you a great overlook on not only how much space each file and folder is taking but also the percentage of the total used space and, for folders, the amount of items, files and folders inside it.
Another great tool is
Ccleaner which scans for unnecessary files such as temporary files and cache files etc. which can hog unnecessary hard drive space.
Also make sure you don't only delete the files but that you also empty the trash bin ;3
As for making videos smaller without loosing quality I'm not sure, you could encode it to a smaller format, but that might loose some quality.