The form you should use may be given to you in a question, but if not, any one will do. If in doubt, \(k\cos (x - \alpha )\) usually works. These worked examples show the processes you'll need to go ...

Trigonometric graphs can be sketched when you know the amplitude, period, phase and maximum and minimum turning points. Learn about graphs of trigonometric functions in this National 5 Maths guide.