Hey,
I am new to the fixed gear scene. I usually ride my Cannondale R400S (and even with that I am not too Bike savy). But I decided to build a fixed gear and really break into biking as not only a hobby but a way of life. Anyway, I already bought a 50 tooth chainring and crankset but I am not sure of what to do with my rear cog. My uncle told me to go with something in the 'teens. I will be doing minimal uphill / downhill, mostly city and flatland riding. Please advise. Thanks.

ride around in one gear on your Cannondale till you find what works, the figure out the ratio you need with a little cross-multiply and divide.

say you find 53x18 a good ratio... just take 18 x 50 and divide by 53 to get the tooth count you want.

For a n00b, go for around 70 gear inches, so an 18 or 19t cog would be good to start.

don't take offense to the n00b part 70 gear inches really is a good place to start, or you can be a fool like me and jump in at 81 gear inches (what I used to ride a SS at) and blow up on the first hill you climb. BTW I ride 71 gear inches now

Thanks.. (I am a noob, so no offense taken)

Noob? I prefer to keep my knees intact.