Hi. Measure the width of the shell where the BB threads in. If it's 68mm, you most likely have an English threaded BB. This is the most common type. If it is a Nishiki this is your threading.

Next you need to determine length. If you want to use that crankset, the manufacturer or velobase might be able to give you the correct length. But realistically, almost any "single speed" BB will probably work. If you get one of these in the 68x110 size it is probably going to be fine. If you have a way to test fit a few sizes, you should do that. A LBS will probably also have what you need and can find the correct size for you. If you want to upgrade slightly you could get one of these. If you don't already have it, you will need this tool to install the BB.

Hope that helps.