Tensioning helps if that's what the belts need. If the belts are all nice and tight (press down pretty hard and the belts shouldn't sink easy) then you probably need new ones. Just hope you don't end up replacing all the belts and then find out you need a pulley or something. But these things happen. Belt replacement isn't hard, but there wouldn't be a DIY for that. AutoZone has repair manuals online, and if they're not of much help to you start with getting an actual manual. I use this manual for most things:
http://aarc.epnet.com/autoapp/9307toy/9307toy.htm (Engine/Accessory Drive Belts/V-Belts).