GMAT Strategy

It is hard to answer that in one post. Different problems need different steps.

But almost always, read the entire question. Convert the question into your own language and visualize what they are looking for. Even if they are not looking for a direct answer, you will see that some variable's value is required to get to the answer. Once you identify that variable, go back to the question and read the part where the variable is mentioned (something like this value is 3 times the sum of a and b).

Write down all given values and unknowns.

that is your starting point.

If you really want to learn more, you should enrol for math classes.
If you dont wanna go for class, take some simple problems and start solving. After a few, this will come to you naturally. Attack each and every area of math and solve lot of problems.

If you ever get stuck, you are welcome to post here and seek clarifications.