Advice is always worth what you pay for it...
1. Clamps. You can never have too many clamps. Beg, borrow, or buy some decent bar clamps.
2. Unless you are good with a table saw, and have a good table saw, get somebody qualified at Home Depot to rip your wood to the width you need for all "non-side" panels. Explain to them that it is critical that it all be the same width, and you don't want the saw re-adjusted until all the ripping is done. Don't worry about getting every panel possible out of a 4x8 sheet (jigsaw puzzle like). Just get the pieces to be the same width. You might waste a some wood.
3. Clamp it all together with the bar clamps. Adjust it until it's square and perfect. Light taps with a hammer can be useful here. Once it's perfect, Apply your mechanical connectors (whatever that may be).
In no time you will have a tight, solid, square cabinet.