You plus two close friends at your apartment sounds like a perfect setting for your first trip. If worst comes to worst you can get into bed and curl up in the fetal position and ride it out. Ideally it would be good to have a sitter who was not taking LSD just in case.
An individual's weight doesn't seem to effect how sensitive they are to psychedelics - at least not to the extent that other drugs such as alcohol, dxm etc are.
You dont know how much LSD is on your one tab and neither do we. It could be none, it could be a few hundred micrograms (a huge dose). For this reason if you want to be very cautious you should start with half a tab. However, chances are that you will be underwhelmed and dissapointed by the minimal effects.
LSD typically lasts for 6-12 hours. You and your friends should read up on the effects of acid AND the other chemicals that are commonly sold as LSD such as the DOx chemicals. If, for example, you are still tripping after 24 hours then you are probably not insane but have simply taken DOI.
Have lots of music lined up, some nice food like fruit, ice cream etc and some light tv shows/movies and video games to play if you feel like it at the time. Generally just looking at the ceiling, sky, clouds etc and talking and listening to music is all you want and need
Have fun