Main Interface
The main interface is instantly recognizable by any Sims player, but it's also so simple and streamlined that someone who's never heard the word "Sim" before will be able to immediately pick it up. The bottom of the screen, directly in the center, has seven icons. Check out the screenshot…
The far left icon, the blue smiley face, and the meter under it is collectively your mood. Your overall mood is dependent on an average of the other moods, just to the right. You cannot directly affect the mood, but it is important. When it's maxed out, your Sim becomes "Inspired," and will earn more money through his actions at home. As your Sim's mood rises, so does the meter, and the blue smiley face will get larger and turn green.
The top-left icon, currently gray with a little silhouette on it, is your social meter. All mood meters cycle from deep green (good) to lighter green, yellow, orange, red, and finally gray. The social meter is the only one you cannot improve in your house.
Check out the Being Social section for more information.
The top-right icon, currently yellow with a kinda-smiley face on it, is your fun meter. This increases as your Sim participates in activities he finds fun. This is modified somewhat based on your personality: an Athletic Sim will not find playing computer games as much fun as a Geek or Introvert Sim, but has other options that can help it.
The bottom icons are, from left to right: hunger (turkey), hygiene (soap), bladder (toilet paper), and energy (person who is standing) meters. Those four are pretty self-explanatory, and can be helped out by objects around the house. In fact, your starting furniture and amenities in your house can help all four of these meters.
These meters can change icons now and then based on their exact levels, but their placement is always the same. To improve any mood, you have two options. First, the classic Sims way, you just need to left click an object associated with the mood, then click the precise action you want to take. For example, if you're hungry, you can click the fridge and choose a meal to eat.
Alternatively, you can click the icon itself, and your Sim will perform an action that's level-appropriate to how bad the meter is. For example, if you click the hygiene icon when it's deep green, your Sim will probably wash his hands; if you click it when it's yellow or red, he'll go take a shower. It's much faster to click the icons than find the objects in question, but you might want to select specific objects to exert more control.
Sims will take care of themselves as necessary. If left to their own devices, they will do actions that will help out the lowest meter, provided they have an appropriate object nearby. For example, in the first screen shot, the Sim's lowest meter is his social meter; left to his own devices, he would seek out someone to talk to. However, there is no one at his house, so he would instead seek to do something fun, which is the next-lowest meter. You cannot turn off this autonomy, but any command you order will immediately override anything he chose to do on his own.
You can, in fact, leave him on his own for several minutes before the game yells at you to take back manual control. However, autonomous Sims never do any action that expends energy (see Making Money for more details), nor will he ever do any action to make money. Still, allowing him to be autonomous for awhile will help him improve his mood without your intervention. Be warned though that when a Sim is autonomous, he's more likely to break any object he's interacting with.