I purchased a game from Steam, but my computer storage is completely full, I've tried many methods to get rid of unwanted stuff taking up my storage, but nothing worked. If you have any suggestions, then please tell me them.) I have an extra USB drive but I don't know if it will work.
You can use your USB drive as an external steam library just like any other drive. But beware, a mere 16GB (on a decent USB) or even 64GB on an expensive USB won't be able to store many games. USBs are also prone to corruption and data loss. Therefore it may be a short term solution but it won't be an effective strategy long term. If you really can't find any old files or data that you can spare you can buy external hard drives, but if possible I would recommend buying a new internal hard drive (2TB hard drives can be as cheap as £50 or your regional equivalent). If you can fit a new internal HDD to act as a steam library and more data storage it will be a very effective long term solution.