Here I present you the two most common problems and their solution.
Problem number 1
Curling. Extreme curling of your work.
Luckily this is normal for Tunisian crochet. Annoying, but normal. You can combat it after finishing it it's not too bad. You can either block it but a round of single crochet helps to get your work flat. No worries.
If it's really really bad try a larger size hook. You need to work a bit loosely.
Problem number 2
Your work is transforming into something diagonal, you are losing stitches!
That last stitch is sometimes so hard to see... I had my share of problems with the little bugger.
The solution is a simple one: when you made your loop on your first row, add a stitch marker to that loop on your needle.
Then, make your chain 1 and work your return pass.
For your next forward pass, the spot where your stitch marker is, is the exact spot where you need to work for the last stitch!
I hope these tips will help you out.
Please do not hesitate to ask your questions/pose your problems!