Hey man, what size tyre did it come with and what size do you want to run?, 2.5, 2.7, or even 3 (they're a bit excessive I think). You could try chucking a mates tyre on or measure the width of the inside of the frame at it's narrowest point then go to your LBS and measure the width of the tyre you want, I'm pretty sure the width of tyres at their widest point is written on the side somewhere. Hope that helps slightly, sorry I can't give an exact answer, good luck champ.
Cheers