What to know before start learning modes

Basic Theory Required
You need to understand how a major scale is constructed. We will be relating each Mode back to it's PMS to work out the characteristics of each mode, so this is really important.
You also need to know what sharps and flats are found in different keys. This helps you find your PMS and to work out the notes in any mode.
You will also need to understand how chords are constructed and how they can be altered to make new chords. This is because modes are played over chords, and without understanding the harmony created by the chords you won't be able to use modes!

Practical Skills Required
You need to know AT LEAST position 1 of the Major Scale, but it will be a great help if you know all five positions. And not just play them up and down, but be able to use it / them. If you don't then I would strongly advise you spend your time getting to know your major scales well before you attempt to use and understand modes. You can learn 1 position
1 position for major scale is (for C scale)