If you're driving, there are several major interstates to St. Louis.
It is probably best to avoid the downtown area if possible. I-270 (red) allows you to bypass downtown, if you are coming in on any of I-70, I-55, or I-44.
St. Louis has an Lambert-St. Louis International Airport, with airport code "STL". All the major carriers service Lambert, but especially American Airlines.
A set of rooms has been set aside at the reception hotel, the St. Louis Marriott West The block rate is $89/night for two double beds. The cutoff for reservations under the block is May 12th. Mention the wedding to get the block rate.
If the block runs out, or if you want to stay at another hotel, you could try the Coutyard St. Louis Creve Coeur. Their rates are $99/night for 1 king bed and $109/night for 2 queen beds under the weekend rates. They also have suites available. The hotel is walking distance to many restaurants, and is about halfway between the church and the reception site (5 minutes drive).