if you have never done shrooms before, and never done salvia before, why in god's name would you try combining them on your first go?
stick with 1/16th oz (~1.75g) for your first time. if you smoke weed regularly, have a joint pre-rolled in case you think you need it during the trip but i would suggest you don't combine anything with shrooms your very first time. just try to get a feel for it.
read the first time tripper thread, read some more trip reports. make sure your ipod is charged or your favorite cds are ready to be played. dim the lights and try dancing. don't forget to stay hydrated and keep light snacks around (apple, mixed nuts, etc). close your eyes and push on your eyelids and see what you can see.
don't trip if you have any unresolved issues in your life right now. the mushrooms can really mess with your emotions and make you feel like you are not in the right place in life. for some people who are experienced with this feeling, it can be put to use and can help catalyze a positive change. but if you are unprepared for the wrath of your subconscious, it can completely devastate you for weeks after the trip. IMO the best time to trip is right after you accomplish something that has a tangible effect on moving your life forward on your chosen path (for students, when final exams are all done)
don't be afraid to meditate. i'm not sure how your friend is approaching the mushroom, but if you have any religious or spiritual background i highly suggest you try meditating. even if you don't know how to meditate, the mushroom can teach you. simply listen to your breathing and try humming lowly to yourself. with practice, you can fly among the stars and time travel.
some people say turn off and set aside your phone because parents have a peculiar habit of calling right at the most inopportune time, but if you're like me then your phone is almost an extension of your hand and you should keep it in case you feel the need to talk to a comforting friend or significant other. since you're tripping with someone else, think about getting another friend to tripsit for you. it is really nice to have someone sober to tell you that the cops are not coming to get you, the fridge is not on fire, etc. experienced tripsitters will also be able to help choose music and ambiance to guide your trip into something that you would not be able to do on your own.
finally, remember that it's just a drug and you may feel crazy for a couple hours. that's why you ate them after all
good luck and godspeed!