If you use your AWS instance for just the bot there is essentially no chance it will leave the free tier and charge you (other than the temporary $1 for identity check), but if you do many other things as well you may end up being charged.
Choose Ubuntu Server 20.04 as your operating system, and the t2.micro as your instance. Both of these are free tier eligible so you will not be charged for them. After you have chosen them, press launch instance.