The way you let the court and other side know, formally, that you are going to defend yourself, is by filing an “Entry of Appearance.” This video discusses this requirement and some other “red tape” like how days are counted.
The Entry of Appearance is important because once you do that, the court and other side must provide you notice of anything that is happening. Before that, you are subject to default without notice. You CAN enter a case without answering it, but you’re still subject to default. So enter your appearance AND answer the petition at the same time – even if you aren’t required by the court to do so.
In the lower courts in Missouri, people often show up for court and announce their presence when called. But if they don’t file an answer, if they miss another hearing they still get defaulted. Don’t let this happen to you.