Most chams want very little to do with humans or any other animals for that matter. Their nature is to blend in and stay as far from the spotlight as possible. But like every thing else there are exceptions to the rule, every Cham is different.
Best thing you can do to earn their trust is to let them settle into their new environment for a week or so without any additional handling. While they are settling in hand feed as much as you can, this will teach your Cham to associate you with positive experiences. Over the coming weeks he will relax a bit, that is when you can begin to try to pick him up. NEVER put your hand above and behind him, nature has programmed him to panic whenever something suddenly appears there.Thats how most predators get them.
To pick him up put one hand on each side of him and below him and slowly bring your hands together while lifting gently. The idea is to slowly replace whatever they are standing on with your hands. You really need to pay attention to your cams behavior while doing this. If he completely freaks out and runs away give him more time. If your cam flat out refuses to allow you to pick them up I suggest you put your hand in front of them and with your other hand have their favorite treat ready to go . Start by just getting their attention with the treat but keep it out of their tongues range, the idea is to force them to walk forward on to your hand to reach the treat.