I will answer for practices in my county. They can vary a little from county to county.
If you are going to try to get late fees, your complaint needs to include that as a separate count - e.g. count 1 for rent and count 2 for late fees. My county has a sample form online, but it is for rent only.
Make sure you have the appropriate number of copies of the complaint. Have it notarized before you go, as you may not be able to easily find a notary once you are there.
Have cash to pay for the fee. In my county the only checks accepted are from attorneys.
Seriously consider having an approved private process server so you increase tremendously your chance of service. Line up who that person that will be before you go. This requires a special form requesting an approved private process server. Overall, the cost will be only slightly more than having the sheriff's office do it.
Also consider having a post or serve notice. At least you know then that you can get a judgment for possession even if the tenant is served only by posting. With posting, you need a separate case for rent.
Possibly you may need a self-addressed stamped envelope.
You won't need a copy of your lease, etc until you have a hearing date.
And in Missouri you don't have to have given any kind of formal 3 or 5 or whatever notice as you have to do in many states.
--70.246.xxx.xx