1) Identify what your variables and constants are.
2) Examine the relationship between them (i.e. positive or negative).
3) The variables always go with "x", for example: Timmy works in an electronic store and makes $60 every day (constant) and additionally, he makes $10 for every computer he sells (variable). Notice the word "every", this hints at what your variable is. So, in this example your equation would look like this: y = 10x + 60
Hope that helps!