In Indiana State courts, people can start a case without using a lawyer. This is called being "Pro se". One of the first questions of a pro se litigant is how they can start a lawsuit.
The first thing to do is be sure you know what it is you are suing for and who it is you are suing. Then you should put that information on paper the best you can, being as brief as possible while still laying out all of the important facts.
The courthouses will generally have a clerk inside. The clerk is in charge of managing the papers that become part of a lawsuit. So find the courthouse, and go see the clerk.
With the clerk you will bring at least 3 copies of your paper, which you can call a complaint, to the clerk. The clerk will file it and assign it a case number, which you should remember. Be sure to get a copy for yourself. The lawsuit is now started, though there is much left to do, you have taken the first step of starting a lawuit.
Oh, read the rules! They will tell you some things that you need to do with respect to your papers and filing them. Below is a link to the State of Indiana's trial rules. Read the local court's rules too, they can be found on the internet at each county's website usually.