Advice needed on bikes!

Hello

I'm a runner who is looking to get into triathlons at a very basic sprint level!

I'm looking to buy a bike that will get me around town, out on the roads keeping fit etc but that I could use for entering triathlons in the future.

I bought a bike last year from Halfords that cost about £300 and it was a "town and trail" bike. Unfortunately it was stolen and uninsured, so I'm back to the drawing board!

Can anyone give me some advice on what type of bike I should be looking for, as I have no idea and there seems to be so many different types of bike!

Any advice greatly appreciated.

Thanks