Do you mean you do not know how in Vegas (technically) or where the audio elements should be placed. Typically, dialog is placed in the center channel.
Basically. dialog: center channel; music: front left and right (stereo) channels The content in the rear left and right" is subjective. Many post audio mixers add a little S/FX to both the left/right front and rear. Some low frequency effects can be added to the sub channel, If you are submitting it for broadcast or a film festival, it is 'safer' to do a plain old stereo mix. If a surround mix is not mixed properly, the down-mix (playback on stereo and mono devices) could be greatly affected, I've read of more that a few nightmares where dialog or other crucial elements were missing when played back on non-surround sound systems.