It depends on yourself really - look around you, what do you have to work with? Got spare parts and tools? How much money can you afford to spend on the bike? Maybe you can get a cheap donor bike from CL (assuming you live in the USA). Then where does the frame come from, is it something from a walmart bike or something that has a bit of value?
Have a look through various build threads and see which bikes you like and then get your ideas from there - read what the people have done to their bikes and what makes them look the way they look. Just gather information and ideas and then get to work! If it would be my bike I would use big'n'littles, drop bars, angled seat post, old style saddle and no unnecessary accessories. But that's me, maybe you want a full-on rat with loads of stuff added to it and über high ape-hangers, or maybe just a normal beach cruiser with a two tone paint job?
If you are more specific about where you want to take that bike then people can help you a lot more.
If in doubt, just get it running (do you say that with bicycles?) and ride the snot out of it !