Depending on what country you live in, both Amazon and Newegg will probably be your best resource for obtaining computer components. There are a few other Online retailers too, like NCIX, FrozenCPU, etc.
Resource-wise, definitely take a look at reddit's
buildapc subforum. Video-wise, YouTube has tons upon tons of videos teaching about the stuff.
LinusTechTips would easily be the top recommended channel I suggest anyone check out for getting information on computer parts and accessories. The playlist I linked above is his "Build Guides & Buyer's Guides series of videos. Lengthy, but definitely worth watching.