Congratulations on your new budgie.
The key to good nutrition is a little bit of everything. Try not to let him concentrate on just one food item. Make sure he always has a crumbled pellet available & then add some veggies, fruit & seed to that.
Its ok to leave him/her alone for a day to settle in, but once the bird is comfortable, you can start taming. Don't rush him, but when puttng food in the cage, hold your hand in the cage to see if he is frightened or curious. If he isn't frightened, you can slowly move a little closer, but if he is frightened, just hold steady for a couple of minutes until he accepts the hand is not going to hurt him. Doing this several times a day will get him used to your hand & then you can slowly get closer until you touch him. If his favourite food is something green, you can hold a bit of it in your hand & see if he will take it from your hand as well.
Good luck. I hope you drop by here every once in awhile to let us know how things are going with you and Bert.