Theory Tips

The first thing you need to work out is the numerical value of the interval. 
Do this by counting the letter names to get from the lower to the higher note. Don't forget to include these notes in your calculation so C to F would be a 4th (C, D, E, F)
Start from the assumption that the lower note is the tonic of a major scale. This gives you the starting point of intervals being major or perfect. Then adjust a semitone at a time. Andante-in-c told me her tip which is to have your hands one above the other and move the top and bottom hand up or down a semitone so that you can see if the interval becomes bigger or smaller.
A semitone smaller than major is minor.
A semitone smaller than minor or perfect is diminished.
A semitone bigger than major or perfect is augmented.
A semitone bigger than minor (if you need it) is major.