Couriers use single speed fixed as they are light, fast and as close to unbreakable as you can get on a bike. Good work out from single speed - the first few weeks will build your legs. Not so great if you live in a hilly area. Don't choose an aggresive cog ratio (you can change that later for more speed) and you will be fine. If your looking at fixed wheel rather than floating try one first - completely different when the breaking is controlled by your legs.
Alternative is hub gears - have a look at the charge mixer - gives you 8 gears in an almost as unbreakable hub, downside is weight - fair bit heavier.