Agree with Sharky; I'm just starting out (5 days in) my basic five tips (I think to myself, as an older person) would be:
1. Realise that it's initially going to hurt; but hopefully you will get through it.
2. Don't be too hard on yourself; it's not going to be easy (unlike walking or swimming, as comparative exercises).
3. Think small: initially do small distances and short duration's.
4. Cycle regularly (ie five days a week; but remembering tip 3).
5. Try to remember that eventually you will come to actually enjoy it (but it will take time and you need to get through all of the initial hardships).
And before all of the above; get a bike (and preferably a helmet); doesn't initially have to be a good one; initially you are just trying to get your legs, heart and lungs going; a good bike can come later when you have decided what type of cycling takes your fancy. Same with clothing; clothing does help, but initially just get your heart, legs and lungs going; then get a good pair of shorts!
So there you go; some tips for a beginner from a complete beginner; shoot me down in flames; any criticisms and thoughts would be most welcome