There are many ways of making a car faster, most of them are either expensive or labor intensive though. Some of these increase power, or you can do something to the transmission, lower the car, change it's shape, or remove weight.

"would it go faster if i put a bigger cog on the back wheel"

no, you need a smaller "cog" or sprocket on the back wheel to up the speed, but your still only looking at a 3% gain in top end speed. try a larger sprocket on the front eg. if you have a 15 tooth sprocket change it for a 16 or 17 tooth. this should give you up to about 10 to 15% gain at most.

if you want a bit more then you'll need to look in to rejetting the carb, free flow airfilter, change the cdi for a race or unrestricted cdi, and a free flowing exhaust to help it all on it's way. but remember that even after all these mods your still only looking at a small top end gain, after all it's still a 125cc. if your looking to bring the speed up even more than you need to find a good big bore kit, but that's a lot of work and not one i'd recomened a novice to install.