Decide what you want it to do. Figure out what it needs in the way of structure ("skeleton"), effectors ("muscles"), senses and intelligence to accomplish that task. (Look at other machines to see how they accomplish tasks which are similar to this one, or to parts of this one.) Find parts which will perform those functions. Assemble/program. Try it. Fix it.. Try it. ... "Lather. Rinse. Repeat if desired."If the robot needs to do more than one thing, develop and debug each one in turn and then decide whether you should simply glue them together or if a better solution exists by combining the functions.Reading descriptions of existing robots, here and elsewhere on the web, should answer a lot of your questions. Hitting your library's electronics and computers section may also find some useful references.