Make sure you know the sum of all the angles in a triangle, this is 180o.
You also need to know the following:
With this information you can calculate the missing angles in an exercise. Sometimes it is nessesary to calculate other angles in a given figure before you can calculate the missing angle in the question.
Make sure you always make a sketch in your notebook you can use to help you!
It can help to just try and calculate as much as possible when your stuck and don't know how to proceed.
Remember to always write down in brackets why an angle is an certains amounts of degrees like in theory C (the image to the left)