If you will be at least 18 years old by the date of the next election and are a legal Missouri resident and U.S. citizen, you are eligible to vote! Find forms and deadlines at the Secretary of State's website.
Please note that if you are an out-of-state student attending SLU, you must choose where to vote (by absentee ballot, if necessary). The Secretary of State's office in your home state is usually a good place to begin looking for information about this process, although it can be slightly different depending on the state.