How do I set up my laptop for dual boot?

I suppose to buy a Laptop and want to install Linux OS(not decided which distribution to use), and I have a large volume of data with the NTFS format drive externally.

How do I need to format my laptop hard-drive? (I mean to which format). And if I need to install Windows on it after that, can I do it? Is it possible? Further, to which format do I need to format my laptop hard-drive, if I want to access the files in the hard-drive from both OSs(Linux and Windows)?

Thanks in Advanced.

You can Install Windows later. There should be no problem in it. There are many threads on internet covering Windows/Ubuntu or Windows/Linux dual boot scenario. This one is from Ubuntu help https://help.ubuntu.com/community/WindowsDualBoot

As far as the question of partition format goes, almost all Linux distributions can handle all commonly used file system types including FAT32 and NTFS. Windows on the other hand doesnt understand EXT3 or EXT4 file systems which are most common on Linux distros. So if you want to be able to install Windows later and want to access all of your hard disk from both OSes then you should choose either Fat32 or NTFS because both Linux and Windows can handle them.

A better solution would be to use ext3 for Linux partition and NTFS for Windows partition and have a third partition for common files/data and format it to Fat32 or NTFS.

In case you buy a new laptop - as you say - you will most likely have some basic Windows OS already preinstalled. You need to complete the setup first sometimes. However all partitions on your laptop will have been NTFS formatted for Windows only use. Some installers give you an option to leave part of your harddisk unpartitioned - very good idea when you plan to install Ubuntu later. If not you will need to shrink your partitions to give an unpartitioned space for Ubuntu. If you have used Windows already it is advisable to defrag your partitions first.

Installing Ubuntu gives you the choice of using the entire disk (thus deleting Windows), or to install it in a dual boot setting in case a preexisting OS was detected. Then you will have the choice which OS you would like to boot into but there is no way switching between both OSs during runtime. By default Ubuntu will format the partition(s) in /ext4 format. You can't install Linux on an NTFS partition like you can't install Windows on a Linux partition. Ubuntu can access NTFS (and FAT) partitions by default. No need to install anything else.

For testing different Linux distributions, and to see which on meets your needs I therefore recommend you installed them in a virtual machine and play around a bit. You could even simulate the dualboot situation in such a virtual machine and learn how to do it before you go for your main system. However keep in mind that a virtualized OS has a much poorer performance as compared to a real installation.

I do not recommend any unexperienced users to install Windows after Ubuntu.