How do I know when to use the appropriate formula, lets say for a trigonometry question.

First of all, I would suggest you that you memorise or find a creative way to memorise the formulas. Personally, I start practising problems of that specific type with a cheat sheat of all the formulas that I will need. After gaining some confidence and seeing that I am able to obtain most the the method marks and the application marks on a particular question, I will remove the cheat sheet and will try to solve the questions. After having the knowledge of the formulas, I would suggest that you start to explore diffrent types of problems (such as that and that - giving examples with the whiteboard or from some past papers). And particurly in maths the more you solve, the more you gain confidence and you are actually able to solve the problems and learning where to plug diffrent formulas, just by having the hard work of solving loads of questions.