There was a UReddit course several months back, those videos might still be up. Where to go really depends on what you want to learn. If you just want to get a general feel for it, Wikipedia's probably the best place. If you want to actually learn the subject, it's going to take some studying. If you don't have any abstract algebra you'll almost certainly benefit from learning some first. I found Dummit & Foote and Artin to both be useful texts for abstract algebra, and Awodey to be useful for category theory.