I am not fully sure on this, since it's been awhile since I've had experience with Sims 3 console edition, but you may not be able to control your household all at the same time. This may be due to the limitations of what the console can handle.
Meaning if you have Sim at home and they're painting for example, and then you take your other Sim to the gym, you may only be able to control one or the other Sim. The not active Sim may end up having to stop what they're doing. While your actively playing the other one.
I am not finding an actual step-by-step guide to how to play the game itself. However, you might have a look on Youtube for Let's Play Sims 3 XBox 360 to get a better idea of how to play the game.