When it comes to framing, one of the things you need to consider is how long do you want your artwork to last for? Your back board and matting are critical to this, and the glass/plexiglass has an effect.
Most wood/paper products are not ph neutral and will damage prints and posters over time. If you use bristol board or even particle board for your backing material your print will disclour over time. Same if you use cheap matting (the stuff that comes in $5 ikea frames).
Non-acidic or 'archival' matting material is fairly cheap and easy to get if you look for it (lots of framing supply places online). For cutting your matte and backing, a metal ruler and exacto/hobby knife works just fine. I've found Michael's to be a huge rip off on both of these. I can get archival 4x matte board for $10 a sheet (24"x36") at the local art supply store, but it is $15 at Michael's, and I can get a 5 pack of exact blades for $2.50 at Staples, but Michael's cost $10 for a two pack. Michael's prices around here seem based around everyone always having a 40% off coupon.
Similarly, you want a plexiglass or glass that will stop UV (ideally while still making your image 'pop', cheap plexi-glass can cut down on the contrast in your poster/print). Framing supply places are a good cheap source, but for even less look at glass-art stores (places that specialize in stained glass and that sort of thing).
Toeinthewater's suggestion of finding an online framing supplier and buying premade kits is probably the easiest method and certainly seems economical.