U. S. citizens may vote in national, state, and local elections if they are at least 18 years old at the time of election, and a resident of the state, county, and precinct for at least 30 days prior to the election. Eligible residents can register to vote any time up to 30 days before an election.
There are several places in Missoula where you can register to vote:
Missoula County Election Office
200 W. Broadway
Montana Motor Vehicle Division
2100 Stevens Ave.