If it's just colicky/gas type upset, then there are a few things you can try. Putting the baby into different positions to find one that makes them more comfortable always helps- my baby's favorite was to lay her across my knees on her tummy, and rub her back while gently swinging my legs side to side. Putting her in a sling and vacuuming sometimes would help too (and you get to clean some in the process!). You can also try mylicon drops, but I found them to be only minimally effective.
If you're talking about throwing up though; by which I mean either spitting up large volumes, forcefully, or frequently (as in more than just within 15 minutes of feeding) then it's a different matter. All babies spit up, but if they're doing it A LOT then you have a few different possibilities.
If you're formula feeding, it could be a problem with the formula. Changing formula types may help.
If it's just recently started (as in a day or so) then you could have a stomach bug on your hands. Make sure baby stays hydrated, and give them extra hugs and cuddles. If they get a high fever, or become listless or seriously dehydrated, call a doctor.
If this has been ongoing for an extended time, and it's not a formula issue, you could have GERD (GastroEsophogeal Reflux Disorder) on your hands. This is when a baby, for whatever reason, has an underdeveloped pyloric sphinchter, which allows digestive processes to back up into the esophagus when the stomach is full. If you suspect this, you should mention it to your doctor- there are a number of tricks s/he can suggest, of which I only know a couple. (Smaller, more frequent meals, and making sure the baby stays in a sitting position for 30 minutes after eating).