can i write my own music on musescore?

Have you downloaded the program at https://musescore.org/en/download? If so, then start entering your song. If not, then download the version for your computer. I suggest that you checkout the tutorials at https://musescore.org/en/tutorials that show the basics. These tutorials were developed under version 2, so there are a few differences - the largest of which is automatic placement of most items - but the basics remain the same. Familiarizing yourself with the handbook is also a good idea. This has been updated for version 3 and will help you find things that have been moved from version 2.