All modern day versions of Windows come pre-loaded with both Notepad and Wordpad. Both of these will give you the ability to write anything you want.
If you want Microsoft Word, however, you will have to buy it as it is not free.
If you like the features that Word provides, you can install any of the free generic Office programs. In this scenario, Libre Office is the best. That is here http://www.libreoffice.org
You can buy a copy of Microsoft Office that includes Word from www.microsoft.com/store. You can download Word from the app store on many devices. You can use Word for free in a browser by creating a .docx file at your www.onedrive.com account.
Assuming you have a computer that has a windows operating system (versus Mac), then Word is a popular and easy program to use. Get the student package, it's less expensive than buying a single software program. Word is the preferred software for many businesses and schools. It can help you to learn skills that are required in the working world.
Brayden has it right. Any text application can be used, but most publishers want .doc, or .docx files submitted. Word, or libreoffice can save as the correct formats. (as can Apple's "Pages" if you prefer a Mac.) A few may want a .pdf file, but they are fairly rare these days and the same apps that will save to the other formats can also save as .pdf. Also most publishers will specify the document format.
download the FREE office software called OpenOffice it does most things pretty well.