What Dan said. If you're flash - to - subject distance was going to be the same every time (say if you set up a booth) and your ambient levels wouldn't change (ie. the sun isn't popping out from behind the clouds every other shot) then you would use Manual flash and Manual mode on cam and set the ambient first and introduce flash fill, this would give consistent shots over and over. But with events. where people move around a lot and lighting conditions change often (inside to outside for example) you're best to use TTL.
I have usually set the camera to manual and spot metering mode and taken a rough reading from the background wherever i'm shooting, that way the ambient is consistent and then you can let the TTL function of the flash do the rest. Then when you move to a different spot to shoot with different ambient lighting conditions you can do the same again to check the ambient using the camera's meter and fire off however many shots you want there. You can even turn the flash off for a sec while you set the ambient. The TTL on that flash is very good so you should get nice consistent fill that way.
Or you can use your camera's AV mode as mentioned and let technology figure out both the ambient and flash levels for you. I've never experimented with the difference in camera metering modes this way so it could be worth trying it with the camera set on centre-weighted Averaging and spot metering modes in a variety of lighting conditions (indoors, outdoors, backlit, front-lit etc.) and seeing what works best before you shoot the party.
I totally agree with bouncing flash off the ceiling too, if it's a white ceiling and not too high you are basically sorted for nice diffused flash shots.
Since you're there to have fun too you could have a play shooting on full manual everything, it couldn't hurt could it? Without wanting to overload you with information! there are a couple of basic rules you can follow to get you in the right place. Always think in terms of flash and ambient and treat them as separate exposures. So shutter speed only affects the ambient. Never go over 1/125s (you'll have to google the reason for that if you're interested). Once you have the ambient set, switch on your flash and adjust the power up or down to get the exposure right, it really is that simple. A couple more tricks.... If you open the aperture up one stop you'll need to compensate by halving the flash power, and the same if you double up the iso. Iso and aperture affect both ambient and flash and shutter speed affects only ambient.
Have a play and let us know how it goes.