First of all, potty train your tama. When you see it moving from side to side and wiggly lines are surrounding it, select the toilet icon. I don't know exactly how many times you have to do this, but I usually get them seven or eight times. Potty times differ depending on age. Toddlers go every 1 hour and 40 minutes and teens 2 hours and 20 minutes. I don't know the exact times for adults and babies are too young to learn.
Second, make sure you feed your tama. Most foods raise weight by one and most snacks raise weight by two. Chocolate cake raises weight by three. The standard pork and beans meal raises weight by two and the standard milk snack raises weight by one.
Third, play games with your tama to keep its weight down. There are two games, Shoot The Bug and Long Jumper. I personally find Long Jumper easier. Your tama loses between one and four pounds depending on your score.
You can change the interior of the room at the shop. It's under remodel. Restock times are 11:00 AM, 3:00 PM and 6:00 PM for food.
Gotchi Figures are extras. You don't need them, but they are fun to play with.
Hope this helps!