I've been teaching myself for at least a year or so now and it's going okay.

I'm planning to start individual tuition soon and I would just like to know if any of you more experienced players have any advice on the most important things an up and coming guitar player should know?

Ideally, I would like to one day write my own stuff on the guitar, do a bit of tricky soloing etc. What are the most important aspects of the theory side that I should learn and how will it benefit me?

I appreciate any advice you may have.

Knowing about chords and scales and how they relate will help you a lot. Also ask your teacher about the CAGED-system, which is a method for understanding how the fretboard works. Apart from that, just learn lots of songs and solo's. It's fun and inspiring and you learn stuff in a musical context.

Along with the basics I learned in lessons, playing along with records was the best school for my ears.

But it depends how far you want to go. Many people can get by with a few chords and a pentatonic (blues) scale.

Learn what you can, and play enough to incorporate whatever's useful to you.

Learn as many scales as you can fit into your head, and learn them in every position - you'll feel very much more at home on a guitar when you can hit the right notes all over the neck (and not in a chromatic style - that comes later when you're a little more advanced, or when you're drunk)

The way I did it was to first learn the root notes of a given scale all over the fretboard, then the corresponding patterns to fit around them. After a while, you'll get familiar with where every note is, so you'll basically just have to learn the patterns.

Learning scales in every position is useful in a number of ways:
1) it builds fluid movement across the neck in any direction
2) you begin to understand the construction of each scale - the sharps and flats and so on
3) which in turn gives you a better, more instinctive knowledge of what each note is on the fretboard
4) feeding on from that, this fretboard knowledge builds your understanding of chords, how they are constructed and how they fit with soloing.
5) you learn how each scale 'feels' - which are sad, happy, mysterious, eastern, western, gothic etc etc, which is an absolutely critical thing to know when you're composing or improvising.

Doing this is hard work, but it really pays off. I apply my usual 20-times rule (play the lot 20 times, if you make a mistake, start again, and keep doing that until you can do it 20 times without error) to learning scales, and I'm still a million miles off having a comprehensive knowledge. But it's extremely satisfying at the same time, which makes it worthwhile.

It's great that you want to understand the theory behind the guitar. It always puzzles me that the majority of players don't want to know this kind of thing, when it is the key to goodness.