TL;DR* No need to take action yet, Windows 10 comes out 29th July and the free upgrade is valid for a year.
Most of us Windows 7 and Windows 8 users are getting “get Windows 10” pop-ups inviting us to register for a free upgrade to Windows 10. If you don’t have a pop-up yet, don’t worry, you will be eligible for an upgrade at some point.
Windows 7 and Windows 8 users will have the chance to upgrade to Windows 10 for free when it comes out on 29th July. Microsoft is trying to get people to register for the free upgrade before it’s released; it’s just a marketing ploy. Whether or not you want to upgrade is up to you, it depends on whether you want the new features of Windows 10.
If you do decide to upgrade, there are two options; and “in-place upgrade” which (supposedly) leaves your files and software on the computer, or a “fresh install” where you backup your data (documents, photos, etc), wipe the hard disk, do a fresh install of Windows 10, install your software again, restore your data. For previous upgrades to Windows it was recommended that you do a “fresh install” to minimize the risk of things not working properly afterwards, and I expect that will be the best option for most people for Windows 10 too. In any case, I would recommend a backup of your data first. Also remember that some older software and hardware such as printers may not work with Windows 10.
So, what should you do now? I suggest you do nothing at the moment, but registering for the upgrade doesn’t commit you to the upgrade (I think). The free upgrade will be available for a year anyway. I would recommend waiting until Windows 10 has been installed by many more people and most of the bugs have been fixed. Then, if you want to upgrade, do so when you’re not likely to need the computer for a week in case there are problems.
When you do decide to upgrade, make sure you take your time, and maybe call in a professional to make sure it goes smoothly.
* TL;DR stands for Too Long, Didn’t Read. The tl;dr label is sometimes used constructively by an author to introduce a short summation of a longer piece, as I have here.