Typically there are three ingredients to a relative motion problem. Something is stationary (ground usually), the velocity of a medium (water or air) given with respect to the stationary object and the velocity of an object which is in the medium.

It is useful to know that

\(\displaystyle \vec{v_m} = \vec{v_o} + \vec{v}\)

where \(\displaystyle v_m\) is the net velocity of the object in the medium measured with respect to the stationary object, \(\displaystyle v_0\) is the velocity of the object measured with respect to the stationary object when the medium is not moving, and v is the velocity of the medium measured with respect to the ground.

A standard problem is a boat that has a speed of 20 m/s in still water, crossing a river (the current flowing at 4 m/s). If the boat is to move straight across the river what angle does the boat have to be pointing in and what is the net speed of the boat with respect to the ground?

I'll let you work that if you like, and you can show me what you have been able to do with it.

-Dan