Make sure you have a friend very close by. Both birth Mum and I were very teary and at the end, we had a hug. Please look after yourself.
As for the meeting - just talk about yourself - maybe what you liked doing as a child, whether you like the beach or countryside, if there are any particular books you liked reading as a child, whether there is anything in medical family history that perhaps your child might need to know about - eg diabetes, short sightedness. If you can - and not too traumatic - about pregnancy cravings, the birth, maybe how you and birth father met.
Just be yourself. Don't worry about being emotional - it is very emotional and it will show how much you love your child.
I would always love to know what DS's Birth Mum's hopes or dreams for him were - what she hoped for him - career wise or otherwise.
Don't be afraid to ask questions too -eg what he is in to at the moment. But also about about the adoptive parents. She wanted to know what me and DH did, how long we had been together, whether we had any children. She also asked why we had "chosen" (hate that word) her son - I tried to be honest but it made me choke up (that was when we cried and hugged each other).
A very hard meeting but so important for your son and maybe it will give something to you, too, and not just grief.
Please please be gentle with yourself and make sure you can go and have a hug and chat with a good friend afterwards.