[MrForbes]

Figure out how fast you want to perform the task, then use that to figure out how fast something needs to move or spin. Then compare that speed to the motor speed, and assume that you want the motor to run at about 80% of it's "free" or maxium rpm, so it will be operating efficiently and not loaded too much. Then calculate the ratio needed to do that.

often you will have some type of chain or belt drive involved, and that means you need to figure out the size of sprockets or pulleys also.

It's a complicated process, and gets easier with experience. If you provide a lot more details about what it is you're trying to do, we could probably help you better. Don't worry about someone stealing your ideas...if they're good ideas, then imitation is the sincerest form of flattery. If they're lousy ideas, we'll let you know, so you can work on finding a better idea. win-win!