i have a question pls help me

i want to know, that when to use the three equations of motion... like there are three equations of motions right? so how we will come to know when to use 1st, 2nd and 3rd??pls explain me.. in a easy language..

The three equations have 5 variables.

u, v, a, t, s

Which equation to use can be decided by determining what variables out of these are given, and what need to be found out.

For example, 

if u, v and t are given and we have to find out a

We have v, u, a and t in the first equation, so we will use the first equation then to find out a.

Do get back in case of a doubt!