There are various ways of doing it... you just have to find the way you like working :)
I don't start in the middle, I start at the top left. I find it easier to count up the aida once and go along and down in colour blocks.
I find the middle, then count up and across to get to the top left. Then do all the stitches in that colour that are near where you have started. Don't do it in rows or you'll end up with colours everywhere!
We can probably help you a bit more if you link to the actual kit you have bought.
There's another way to do your stitches too. It doesn't matter which way you do them, just do which is more comfortable for you and stick to doing them one way as it will look better when you have finished it.
Start at the bottom left hole, out through the top right hole - if you only need one stitch in that colour then come back through the bottom right hole and across to the top left hole. If, however, you need more stitches in that colour then do all the 'bottoms' first then the tops... so left bottom to right top, then come out the hole underneath the top right square and that's your next bottom left take it across to your top right, then back in the square under that and you either have your next left bottom or your first 'return' thread.
So you have /////// then you do the 's going back along the row and you get XXXXX
Then if you have some underneath you do those...
XXX
XXXXX
//// and back \\\\
When you have finished a block of colour just run the thread under your finished stitches and if there's a dark colour next to it, leave a small tail and go over it again, but if it's a light colour next to it, just cut it off.
It sounds complicated - but it's actually really easy - don't despair!