Congragulations on your new tegu. You got (IMO) the best tegu out there (the b&w argentine) and especially a baby one which means you'll have a lot of time to work with him until he becomes an adult. Plenty of time for him to get used to you.
A 20 gallon tank isn't going to be big enough. I don't have a camera so I can't take pictures of any of my tanks, but my adult tegu (about 3.5 feet) is kept in a reptile tank (3 sides wood, front is glass, and top is a metal screen which opens) that's about 6 feet long, 2 feet wide, and 1.5 feet high. My tegu (also argentine b&w) looks very comfortable in it. I take him out most of the time whenever I'm home though so he gets his excercise and fun outside of the cage most of the time.
For taming you'll definitely need to handle him daily. Dedicate something like 15-30 (or more if you have the time) minutes per day to handling him.
I got a new baby savannah monitor the other day and I handle him at least 2 (with work it's tough) times per day, the easiest way to get him used to you (IMO again) is to go watch TV and have him with you.
While I relax and watch tv I am holding my savannah in one hand (actually I don't hold him at all, he's never tried to run away) while gently petting behind his ears which he usually falls asleep soon enough and 20 minutes later I get up and go back to the cage and he doesn't wake up until I put him back in, and doesn't want to let go when I try to put him back in.
Find the spot where your tegu likes to be pet/scratched, usually behind the ears is their soft spot. I got a juvenile blackthroat (as aggressive as you can imagine) to actually push into me while I was rubbing behind his ears. He'd close one eye and lean into the side being rubbed.
If you handle your tegu daily there's a good chance by the time he gets to be an adult (4 feet long or so) you'll have him what some call "dog tame".
Also if you haven't already, you'll need heating sources for your tegu. You'll need both UVA and UVB. Usually you'll see (in pet stores) the heat lamps and then the basking bulbs, which all you need is one and that'll be your UVA heat source. On the other side of the tank you can get one of those longer lights you see (that look like they go on a fish tank) - the box will say something like "7% UVB" they are a bit pricy (I paid $30-$40 for the ones I have) but it's for the health of your tegu. As an alternative to that you can get a second heat lamp and get a UVA/UVB lightbulb ($20 at PetsMart) and use that instead. But for the price of a heat lamp and the UVA/UVB bulb it comes out to $30 so it may be better just going with the longer one.
You may already have all this stuff though.
Feeding you can use crickets and pinkies. I've never been able to get my adult Tegu to touch fruits or vegatables but you can also try offering small servings of fruits & vegs. (With the main food source being crickets and pinkies, and eventually fuzzies all the way to adult mice and rats as they grow.)
You may also want to get the reptile calcium powder you see in pet stores and sprinkle that on the crickets. I use it on all my reptiles.
Good luck with your new tegu, if you can dedicate just a little time every day it'll pay off in the long run. Sometimes it seems like you don't really want to handle your lizard (actually I'm speaking from my own side) but if you slack off one day, then you might slack off and not handle him 2 times next week, etc. and it really does take daily handling (for as long as you have them) for them to get to that "dog tame" stage. I've pulled it off one time in my life, with a savannah monitor. I think out of the 6 or so years I had him (since he was a baby) I may have gone 5 days total out of all that time without taking him out of the cage. It really did pay off because he was the tamest reptile I've ever had in my life.
Have fun!
