My opinion about most important things for every, not only first time trip, are:
1. Be with sober or experienced/educated person you feel comfortable with.
2. Have nice setting and be sure you have enough time to trip (for the first time, it's better to have really more time that you can enjoy, not worrying about "What if it will not pass out after 10 hours").
3. Make plan for all the time of tripping about what you can do, how you can be active, if you are not able to just sit and relax (like me. I still have to do anything and tripping amplify these effects for me, so after few hours I get bored, even while I'm still tripping, and that's pretty unconfortable).
4. And as others said, educate yourself and while we are on harm reduction forum, always test your LSD if you can't be sure what it really is.
Enjoy!
- MHF