There's not a real lot to advise on. Being your first date, you don't need to worry at all about whether or not there's lots of making out, you know?
Wait until you're pretty comfortable before you make any moves, there's no point in feeling bad because you rushed for a kiss or whatever. Hold Cassie's hand when you're walking to show her you care, remember to smile a lot and if you want the date to go well, make sure you pay
her the most attention. It's fine to have a good time with Michael and Morgan as well (don't ignore them outright XD), since they're you're friends and are there with you as well, but you want your lady to feel like she's the most important person to you.
That's probably about all I can say. If you wanted to pass on any suggestions to Michael, just make sure he knows to give his main attention to Morgan, also. Everyone likes being important. Even moreso on a double date.
Oh, and don't feel weird if the girls go off together for a little while. For some reason they seem to do that, so it must be fairly normal. Especially the "bathroom migration", when they all seem to go in together and return together. *shrug* (sorry, I don't do it myself, so I don't understand that very well, but I see it all the time)
Most of all, have a good time. Hope it goes really well ^_^