Hunger

Pretty self explanatory, direct your Sim to cook a meal or grab a snack. Meals are most filling if it taken from the best fridge and cooked on the best stove available by the Sim with the most cooking skill points. Your Sim can call for pizza, but it's not nearly as filling as a home-cooked meal.

Bladder

This bar will decrease gradually on its own but eating and drinking coffee/espresso will lower it dramatically. Make sure toilets are easy to access, and that you get your Sims there before the bladder bar turns completely red and they wet their pants (and the floor) which drastically reduces their hygiene bar.

Hygiene

Have your Sim wash his/her hands after he/she uses the toilet, and give them shower or bath before the bar gets below half full. The big, square bathtub is the best to use.

Comfort

Your Sim's comfort meter will gradually decrease as your Sim walks around doing things and if it gets low, he/she will get quite cranky. Direct them a bed, or to a comfortable chair. The hot-tub and relaxing in the love bed (LL) are also great.

Social

Without interaction with other Sims, your Sim will get depressed and cry a lot. Make sure to buy a phone and greet all visitors so they're available to call when your Sim needs some company.

Energy

This bar will automatically drop throughout the day and can be replenished by sleeping in a bed, napping on the sofa or drinking coffee/espresso. A Sim who runs out of energy without making it to a bed or sofa will pass out wherever he/she was and gain energy very slowly.

Fun

All work and no play makes for a grumpy Sim. You can improve the fun meter by directing your Sim to something enjoyable such as playing pool, watching TV, playing games on a computer etc.

Room

This is a measure of what your Sim thinks of the room he/she is currently in. If there are dirty dishes, puddles of water etc, the room rating will be low. To increase the room rating, buy some decorations such as statues and paintings. Oh, and clean up the mess asap!