1. How can I remember the three different formulas involving speed, distance and time?

This little triangle may help.

Cover up the item you are trying to find and what is left is the calculation to do

2. How do I change km per hour (km/h) to metres per second (m/s) ?

Multiply by 1000 to change the km to metres and then divide by 60 to change the hours into minutes and again by 60 to change the minutes into seconds.

3. Are speed and acceleration the same?

Acceleration is the rate at which the speed is changing
i.e. If the acceleration is positive the object's speed is increasing.

4. What is the difference between speed and velocity?

The velocity of an object incudes the direction the object is moving in whereas the speed does not.