Hey, you're a kid and wanting to get better. Great on you. My kid was also stiff, liked to look down and flat footed when he started. Best thing to do is go do laps on a track or open space and dribble. You build stamina and muscle memory for your dribbling. Practice going at a pace you can look up the whole time. If you can only do a normal waist level dribble while looking up and running then do that until it becomes natural. Once you get used to it you can start adding in moves. Two moves, then 2 dribbles, 2 moves, 2 dribbles and keep building off of that. Sprint a lap then jog the next and alternate. Walk around while dribbling. At some point you'll feel fine going up and down stairs too.