Don't laminate it!!
Laminating is a non-reversible process, and plastics alter with age as well as slowly releasing small amounts of acidic gas. This would mean your letter will decompose insode the laminate pouch and nobody in the future could reverse it.
Like others have said, scanning this letter is the first step, then put the original away, inside a neutral clear pouch. Lots of this kind of product exists especially for the 'scrap-book' craft industry. Or fold acid-free tissue round it and put it in a clear flat folder. Above all, store it away in the dark and somewhere that doesn't get steamy - don't expose it to more light if you can help it - and avoid henadling it / letting others handle it as the urea from skin will cause further damage. It's own little box is a good idea, saves it getting banged about in a drawer.
This is pretty top-of-the-range advice for conservation but this letter is obviously valuable to you, so it's worth taking these precautions. However the papaer is not the sort that will last centuries - unlike say linen rag paper - so it might not last the next hundred years in good condition.