So you have square-taper BB, meaning "any" common (Shimano, etc) crankset should work. This page has some info on different types:
Dimensional standards for square-taper bicycle bottom-bracket spindles, and the limits of interchangeability among them.
www.sheldonbrown.com
yours are probably JIS
A glossary of bicycle terms with extensive internal cross referencing. This is spread across 30 different files.
www.sheldonbrown.com
If the frame sticks out a lot near the BB where the cranks or your feet might hit it, you might need "wide" cranks, that bend out from the chainrings more than usual, but I'd avoid those if you can since they make the "Q-factor" wider, which means having your feet farther apart which for most people (that don't have really wide hips) can cause extra wear on the rider's joints, etc., and be uncomfortable or even painful, depending on the rider and usage. Some info about various details:
About bicycle cranks, and how long they should be.
www.sheldonbrown.com
Useful dimensions and conversions for the cyblist/mechanic
www.sheldonbrown.com
A database of bottom bracket sizes for various bicycles.
www.sheldonbrown.com
If your bicycle has a three-piece crankset, as most do, the first step in overhauling your bottom bracket is to remove the cranks (arms) from the ends of the bottom bracket spindle.
www.sheldonbrown.com
Crank length you can decide based on your usage and preferred cadence, leg length vs seat height, etc. If you've ridden other bikes, then you can guess what crank length to use based on which of those worked best for you and what their crank length was.
Chainrink spacing / placement on the cranks should be so that you get a good chainline to your rear sprockets on the wheel; if you use some gears more than others or don't even use some of them, you can line up the front to rear so you get the best (straightest) chainline on the most-used combination.
Some helpful info when you get to installing them
Article by Jobst Brandt about installing square-taper bicycle cranks
www.sheldonbrown.com