I need help on Buying a Bicycle
This is a sticky one to tackle. Firstly, we don't know the terrain you're riding in, we don't know if you're going to commute to work/school/college/university and we don't even know exactly what your budget would be. As for a Single-Speed, why specifically? You could lose weight on a road racer or a mountain bike, or even a BMX. 

Anyhow, if your budget is low, you can look on eBay and try your luck, or you could perhaps buy a bike from a local bike-shop that has been refurbished. I personally always buy refurbished bikes because they're a fair bit cheaper than new. 

For a cheap bike, don't go for ranges like Sports Direct or Tesco's or any other superstore bikes like that, as they're usually badly made and have little to no grease in any bearings, which is very bad! Just look at BikeMan4U on YouTube to see what I mean. 

A fixie with a Flip-Flop hub would be good, as it gives you a choice of what you'd prefer to run around on.