Just got my new single speed put together a few days ago, a Dawes Streetfighter. I'm loving it so far but the gearing is too tall for my ***** ass. It's at 46/16 freewheel right now. I want to try an 18t freewheel cog and also would like to replace the fixed cog on the other side with a freewheel 20t in case I ever need to climb something real steep while I'm carrying extra weight. Changing cogs is something I've never done before. Is there anything else I'll need to replace the fixed cog with a freewheel 20t and should I buy a tool to do it myself or take it to a bike shop? Also any recommendations as far as what brand of cogs I should go with? Thanks.