Please edit your profile to show your location - you can fill in a distro when you get one.
DELL offer gnu/linux laptops in the USA.
Lenovo have pre-installs in the UK, and, IIRC, Europe.
Acer do pre-installs on some of their range, in some countries, as specials - though I understand that much of their 3rd-world sales are gnu/linux based.
So your pre-install options depend on where you live.
Besides that - choosing a laptop is the same as any HW. Go to the store, find a few with the price/features you want, google the models with "linux" in the names. If it's six-months old, it will likely work well since there has been time to reverse engineer the troublesome bits, however, many bleeding edge systems are now engineered to work well with linux.
Just be aware of fancy vendor systems in the bios - these are usually there to prop up deficiencies in certain proprietary offerings, and are seldom useful with linux. You'll be switching them off.
When you go shopping, you can help the cause by asking all salesfolk about gnu/linux compatibility. Take a live distro with you and ask if you can test some machines with it... imply that a machine is deficient if this won't work. Also ask about the MS Windows refund policy (they won't have one - you are raising awareness).
The more people we can get to do this as a routine, the more pressure retailers will have to reconsider their options.
Dual boot is possible, but usually not needed. At most it will be so you can play Windows games.