how do I preserve the wood for raised beds

Hi Liz222 - As no-one else has jumped in (very unusual for this site!!) I'll give you my thoughts for what they're worth - I'm not an expert! Use whatever preservative you want to but line the inside of the wood surround (and the base if you're building on concrete) with plastic sheet. This will a) stop anything nasty from leeching into the soil and b) it will protect the wood as well.
If you're building on concrete, don't forget some drainage holes in the bottom of the beds, raise the beds off the concrete a few inches and have some sort of drain-away for excess water. Hope this helps a bit ...

Thanks for that, I had wondered if I should line them. I will be building then on the earth.
They are going to go on what is now lawn and I don't know if I sould dig up the grass or just put the compost on top? What do you think?

Personally, I'd say leave the grass as it'll die (and become compost!) once it's underground.