Go to the appropriate Small Claims Court (some even have evening hours). You can't just go to any Small Claims Court, as state laws may require you to use only the one where you reside, or where the person you want to sue resides, or where the accident occurred, or where you bought the defective merchandise.
Many of the Court Clerks are very helpful, and will tell you if you are in the "right" Small Claims Court. Ask the Court Clerk for the appropriate form. You'll have to fill out the form which may be known as a "Claim Form" or "Complaint" or "Claim of Plaintiff". You may also have to pay the Court a filing fee to start the action. (If you win your case you MAY be able to recover the amount the Court finds you are owed PLUS the filing fee and certain other court costs.)