Doing so is an easy enough process–fill out a card, pick a party affiliation (if any), and turn it in.
There are many different places a person can go to register: county clerk’s offices, high schools, and government agencies (food stamp offices, social services, etc.) are just a few examples.
And votes do count–Fayette County Clerk Don Blevins cites the 1974 election, in which five district council members were decided by 40 votes or less…two of which by three or four, respectively.
For more information on how to register to vote, you can visit this website.