I wouldn't say replacing the cranks and crank arms is the first step. Before you do that, you need to figure out what you're going to do about chainline. If you're going to buy a new set up fixed gear wheels, that will determine the chainline and which crankset and bottom bracket you should get. If you're going to respace and redish the existing rear wheel or use a set of spacers and a cog that slips over a freehub, then you can set the chainline to whatever you want and you have more freedom with your crankset bottom bracket choice.

Post pictures of what you have -- close ups of cranks and the rear hub -- and you'll likely get more helpful advice.