When creating an RDS instance through the console, you can select the "Multi-AZ deployment" option. Alternatively, you can use the AWS CLI:
aws rds create-db-instance \
--db-instance-identifier mydbinstance \
--db-instance-class db.m5.large \
--engine mysql \
--master-username admin \
--master-user-password password \
--allocated-storage 20 \
--multi-az