Your choice of a pull up bar will very much depend on the set up of the house or flat you live in. For most people, this will mean a doorway pull up bar. However, wall-mounted, joist mounted, or ceiling bars are usually sturdier, and they offer the possibility of moving on to muscle ups as you progress. I recommend the ones from , which offer a range of all these different types.
If you do not have a pull up bar however, there are a few alternatives that can offer a temporary solution: