It really depends on how much waste you need to process. Considering the kinds of questions you are asking, it would be best to start with a small system with about 1 lb of worms, and grow from there. That way if you make mistakes (we all do) and you lose some of your herd, you won't be out the expense of a larger number of worms.

A good starter bin can be made from a Rubbermaid tote, about 10 to 14 gallon size, and 1 lb of worms. In time the population will double, and maybe quadruple, depending on conditions. By then you will have a good feel for managing the worms, and you can decide if you need a larger system.

If you can find a copy of Worms Eat my Garbage at a local library, it will answer a lot of your questions.

Thank you sbryce

I'll 2nd sbryce's suggestion. A pound of worms in a small bin is a great way to start. I'd emphasize a *small* bin because:
1) the worms have a better chance of meeting & mating in a smaller area
2) it should help avoid overfeeding (not likely to put 5 gal. of food in a 10 gal. bin).
You can buy a 10 gallon tote box (Rubbermaid or Sterilite) for ~$4 at most big chains. The link below is just one of many DIY worm bins.

Here is a link that might be useful: DIY worm bin

I'll suggest buying three bins.
1) the worms live in.
2) Under the first bin to collect the water that leaches out.
3) to create a second tray for harvesting purposes.

If you don't use any aggregate/screening at the bottom of your bin as described in the post that plumiebear linked to, and drill lots of 1/4 holes in the bottom of the bin, you can set a bedded and fed bin on top of your active bin, and the worms will migrate up. In 2 or 3 months the lower bin will be almost worm free.

Here's a description of a stackable (multiple bin) system. Again, this is just one of many variations.

Andrew

Here is a link that might be useful: rickd's stackable worm bin