Firstly, I can't believe some of the stupid responses above. For what little it's worth, my sister is in her 50s and can't ride a bike though in her case, she has some balance issues coupled with fear issues (I think landing in that rose bush as a kid has something to do with it).

For the bike, buy whatever you like and feel comfortable on. While there's a lot to be said for buying a hybrid or similar, you may not like them. Fat tyres or skinny tyres make no real difference. About the only advice I'd give is to shy away from full on racing bikes because these are less forgiving that some other bikes, but there are plenty of bikes that look like racing bikes but are designed with more stable geometry and hence are easier to ride. You will find it easier to learn to ride on a bike that has you sitting more upright, and for that, read my thoughts below.

Buy what your heart tells you to buy, NOT your head - my biggest disasters have all been carefully thought out before hand. The only caveat to that is not to buy an expensive bike for the simple reason that cycling may not work for you. On the other hand, if you love your bike, you will ride it more, if you're secretly disappointed, you won't.

Learning to ride is relatively simple if you take it quietly. Set the saddle low so you put your feet on the ground while seated, then raise it as you get more confident. Having a friend help is good, especially someone to go riding with you but you don't have to be, there's nothing wrong with being a lone wolf.

An alternatives to buying a new bike would be to find a bike coop near you. You may be able to pick up a very cheap bike and maybe get some help learning to ride. You might also consider donating the bike back once you're stable on the bike and buy a nice new one. If you're lucky enough to have hire bikes or a public use bike scheme, make use of that to get you riding.

Best of luck. Let us know how you get on.