SC13 has four basic motives to fulfill - think the Sims meets SS13:
Hunger is maintained by eating food. Go to the Pub or try a vending machine.
For Thirst, there are water coolers around to keep you hydrated and vending machines that stock beverages, but you can also head to the Pub and ask the bartender for a drink. Bear in mind that high amounts of ethanol will dehydrate you.
Hygiene is maintained by washing your hands and taking a shower. Make sure you don't have anything in the uniform or suit inventory before taking a shower, and walking instead of running will prevent slipping on wet floors.
For Bladder, when you eat or drink, eventually you have to go. Toilets are found in the restrooms in various locations around the ship, along with showers or bathtubs in some of them.
Although there aren't motives for fun, comfort, social, or environment as in Sims, you will find that positive interactions and clean environments generally provide a positive Moodlet, whereas negative interactions and filthy environments generally cause a negative Moodlet.
Not maintaining your motives will prompt occasional messages in your chat window (e.g. You feel faintly hungry.) and will persist until the motive is back above a certain threshold. Certain objects, reagents, and activities will affect motives, e.g. being covered in dirt will reduce your hygiene. Failing to stay hygienic will make you terribly smelly and gross. If your uniform or clothes are dirty you can find a laundry machine and stuff them in there.
Motives below 25% also cause a negative Moodlet, discussed in the Sanity and Mood section, whereas motives above 75% cause a positive Moodlet. The Moodlets get more dramatic at 0% and 100% respectively. You begin at 50% in each category.