Music writing software
The two most common sheet music editing programs used by professionals are Sibelius and Finale. These are both large, sophisticated programs that work very well but are very expensive to purchase (up to $600). Both companies offer a free trial version that can be used for a limited time. Finale also offers a simpler version called Printmusic which is cheaper, and a program called NotePad that has even fewer options but doesn't cost anything. Sibelius has a simplified, cheaper version called Sibelius First.
Two free and open-source programs which are easy to use are MuseScore and Denemo. They have fewer options than Sibelius or Finale but can be used to prepare simple scores.
The program Lilypond is also free and open-source, but has no graphical interface (music is entered by writing codes in a special text file, like how web pages are built using HTML). This software is powerful but may take a long time to learn if you're not already familiar with working with computer languages such as HTML.
Another option is the online sheet music editor NoteFlight, which is sort of like Google Docs for music. You can use it to create up to 10 songs at once for free, or pay $50/year for more.