h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

First of all, you need to play an instrument. I recommend the guitar, it's easy to learn. All you need to know is power chords (which can be found in the guide) if you can hum a progression, just find the chords that match and, voila, you have a song. Next you need the right number of friends. Too many friends will mean you have an active social life and consquently no time to practice. Too few friends will reduce the probability of finding some that play an instrument and are interested in forming a band. 2 people are the bear minimum to be called a band. I would recommend a guitar-guitar setup or guitar-drums for a 2-person band. That'll give you enough to work on for a while. Practice playing together until it sounds good, whatever style you choose. Then you can add more members, a bass player for example. A Guitar-Bass-Drums lineup forms the core of a rock band. You can add another guitar or a keyboard for a thicker sound, or more exotic instuments as Led Zeppelin did with the mandolin, Jethro Tull did with the flute, or Nirvana did with the Cello. When all your members sound good together you can start playing gigs, but someone else can tell you about that as I've suddenly run out of steam.

The problem with learning to play the guitar is that everyone does it so it more difficult to get in a band.

If you are a drummer, bassist or singer you will find many more offers.

Agreed! Competent drummers are not two-a-penny and they are always in demand. In my experience though, guitarists are ubiquitous but very good talented ones with great and varied ability are very very hard to find. Any one can play guitar, only few bother to play well.

Nonetheless, I believe guitar is a good way to break into playing music. I've seen countless would-be rockers try to learn the bass straightaway cold, and give up out of frustration. The guitar is an easier instrument that can coax out a latent musical genius, or at the very least, put one on the path to critical appreciation of music that will enable one to learn more difficult instruments.
Or you could always bash the drums like a maniac.

Can I draw your attention to the 'How to Fail as a Pop Group' entry - http://www.bbc.co.uk/h2g2/guide/A593714

Yes, I know it's mine, but it does cover some of the same ground as this topic.

Totally agree that learning guitar is a great start. The good thing about playing the guitar is you can learn an entire song that when played solo sounds good. The drums sound rubbish on their own, you need a band to properly enjoy a kit.

I thought that there was too many aspiring guitarists to ever get in a band by playing the guitar. So I learnt the sax. I spent many of my formative years trying to persuade all of my band-playing frends that what their band really needed was a sax player. Never really took off.

That's a real shame, I've spent years looking for a decent horn section to put together a proper soul band. And I've never found more than one sax player ot trumpeter or indeed any sort of horns. I was trying to do a Zappa tribute act at one point as well, but fell at the same hurdle.
Guitar is a good place to start, but Piano will give you a better musical grounding to work from.
Finding a band to work with is very difficult. I've always found it more important to find guys you can go out on the with than great musicians. Cos you can always practice, but if you're a total divot, this will never change. The best bands have all been groups of mates before they ever picked up an instrument (e.g. The Beatles, The White Noise, Van Halen) and those that were really just a load of great musicians all end up in tears (The Velvet Underground, first Metallica lineup with Dave Mustaine, King Crimson). So as well as finding people who play well, you need to find people who can play well with you.
Not so easy, I have only just gone back to playing in a band again after a year off recording solo stuff (which is a lot easier, you only have to punch the information into a drum machine once) Because the arguing and different approaches eventually get too much.

Oh, screw the guitar, bass and the drums. What you really need is accordion or hurdy-gurdy (I don't know if I spelled that correctly, but who cares?), something to really set you apart. There are so many bands with guitar, bass and drums, but you could be a breakthrough pioneer in hip-hop bassoon or something...