As far as convincing your mother, I would start with a simple question. Is she scared of reptiles? Now if the answer to this is yes, that doesnt necessarily mean no to a lizard. It just means getting her to learn about them in some form. If you do your homework and then make every effort to show her the documented information that you have on them and that you can come up with a plan on keeping your tegu and how you will feed it and when and how often you will need to clean its cage and so on, she should see that by doing your homework, you are learning to be adult enough to handle your own pet. Just dont fall into the trap : "Well if you do good with this kind of pet, we will get you one of those". All too often you end up with the pet that your parents want you to have instead of what you wanted to begin with because they think its a better pet. My honest advice on that is to tell them at that point, I have studied the ownership of this animal and not the one you want me to get, so this would be the better of the options. When it comes right down to it, your parents are going to be scared to get you a reptile because of two fears; 1) they will be afraid of the animal getting out and roaming free. ( the answer to this is to show them that there are now cages built that can keep them contained beyond a shadow of a doubt or you can tell them that you will get to work building a custom cage that will be escape proof) 2) They are concerned that they will become this animals primary care giver. Myabe not now but what about when you move out? (so the answer to this is to show them that you have the skills and desire needed to take care of "your" reptile!)
Also dont wait for them to ask you any of these questions, if I were you I would actually set up a "presentation" on how you need this particular animal as your pet and how it will be an important part of your family even after you move out. Make it good though, parents will usually only hear this presentation one time and then their minds are made up. So if I were you, I would do homework and take notes, get pictures, caging requirements, feeding schedules, dietary needs, so on and on. Make it a valiant effort and they will have no choice but to see that you are at the very least "serious".
Sorry to be long winded and wish you luck!
