How can I choose a MOSFET based on Rds? I have understood all the parameters except this
The MOSFET with the minimal Rdson is not always the most suitable. Those with a very low Rdson have a large Qg gate charge. It means that they need a powerful gate driver if you want to do PWM. A higher gate charge makes turn on/off slower, thus higher switching losses. The good MOSFET is the one with the lowest product: Rdson * Qg

The total loss of a MOSFET is the conduction loss + switching loss. If your application needs only to turn on/off a motor, then the MOSFET you choose will suit your demand.