If you're not stretching rubber bands across a cardboard cigar box, it's not all that easy to make your own guitar-type instruments. You have only to watch a video of someone making a guitar to notice that it involves some advanced and specialized wood-working. The body must be built up and finished, and the whole process can take months. However, if you're talking about a banjo, you don't necessarily need to build up the body: they usually have more in common with the body of a drum than the body of a guitar anyway.
Instructables user TimAnderson has created an awesome tutorial for a cookie tin banjo. You won't get out of wood-working: you'll be making the rest of the banjo out of an old bed-post, and creating your own tuning pegs. But it's still much easier than building the body of a guitar.
You can use Anderson's previous tutorials on the subject, linked from the project page, to build other components of the banjo, like the tuning pegs. And when you've finished creating your own banjo, you can visit the Banjo Hangout. They'll give you some ideas for what to do with it.