I just bought a Specialized Langster off Ebay. I haven't ridden it much yet, but the gearing seems a bit too high. I had knee problems when I was young and don't want to aggravate them. Also, the bike is currently set up as a single speed and I would like to change it to a fixed gear.

I have been searching and reading and it looks like there are 2 different thread systems and cogs are spaced differently. I can't stand bike noise or grinding. What cog would work well on the Langster? Also, what tools should I get to remove the current cog/freewheel and attach the new one?

Thanks.