Short answer, this isn't going to be easy.
Your first decision is abut Timespan. Do you intend to lose weight?
- If no, you're going to have to buy a bike rated for your weight.
- If yes, buy a cheaper bike and expect to break some parts of it. Used is totally fine.
An average off-the-shelf bike might have a load rating of 90 kilo (200 pounds) or 110 kilos (~250 pounds) and is therefore unsuitable for you at this time.
FAT BIKES are not designed for fat riders - instead the name comes purely from the 4~5 inch (100-125mm) tyres and they are intended to float over sand and loose surfaces without loosing traction or cutting through. A couple of fatbikes I found were rated to 200 and 250 pounds respectively!
I'd recommend a used bike with these features, and why:
- A Rigid mountain bike (also known as a hybrid or commuter) because it has no suspension components to fail on you.
- A bike with 26" wheels because they're common as dirt, and you're likely to use up a couple.
- Commuter tyres which are relatively smooth (the knobbly ones are really only for offroad, and riding them on the road makes it less fun. This can be an upgrade after you get the bike.
- Steel frame is fine for your first bike.
- V brakes not calipers and not disks (because disk brakes will make it harder to find replacement wheels when they bust)
Your main breakages will be spokes and punctures. So as the need arises you will have to learn to patch tubes and change spokes, and true a wheel. Certainly a bike shop will take money off you for doing this work, but its not hard and in the long run buying the tools will be cheaper than shop fees.
High spoke count wheels would be the single best thing you can find. MTB wheels tend to be 32 spokes, with some 24 or 28. Ideally you'd use a wheel with 36 or 40 or even 48 spokes purely to increase the load abilities. Double-walled rims with eyelets for spokes are a great idea too. Wheels built for tandems would work well on your bike, if they are the same size. Heck you can even ride a tandem solo!
If you prefer to buy new, I'd avoid buying on-line for this - you would benefit from a conversation with your local bike shop. Whereas a $50 or $100 used bike is usable and disposable once you've worn it out and ready to buy a better bike for your new weight.
If your weight is unlikely to decrease (perhaps medical reasons) then you might need to look at a trike purely for the increased carrying capacity of two rear wheels. But they're less fun to ride and not cheap.
Don't take offence, I'm not trying to be harsh here - I started at over 110 kilos thanks to a sedentary office job and too many pies.
Since you're starting out, another tool that is very useful is a way to record your progress. If you have a smartphone, look at http://www.strava.com/ for a recording tool that tracks your location and speed, and plots your times over user-defined segments. Ignore the leaderboard - you're interested in your progress over time.
Remember to get padded shorts - you might think your backside already has padding and that's why you're riding, but its not that simple.
Links to read http://chicargobike.blogspot.co.nz/2013/05/youre-not-too-heavy-to-ride-bike.html