Most any multi-geared bike can easily be converted to a single speed. Remove all but one front chainring, if you have a freewheel hub remove all but one cog from the freewheel body (This can be a bit tricky) or if it's a cassette hub use one cog and spacers (Which chainring position and cog position to use will depend on the chainline), remove the derailleurs (If the frame has long dropouts you're OK but if not get a chain tensioner and mount it in place of the rear derailleur, or simply use the rear derailleur as a tensioner), size the chain, and voila! Two to one gearing (Chainring to cog) is a good place to start (Usually the available chainrings & cogs will allow a good enough 'starting out' combination), i.e. 52-26, 42-21, 34-17, 38-19, etc. Good candiates for a "first" single speed are 20 year old road or rigid mountain bikes with long, horizontal dropouts and rear cassette hubs. They are cheap & plentiful.

I've been riding an MTB rigid frame/fork based single speed with a Surly chain tensioner and freewheel rear hub (A five speed body with a screwed-on third position cog) for six years and recently built up a 700c single speed frame with front brake only (In comparison, light & responsive and very clean looking).