I would like a software that I will be able to write music sheets/scores.
Nothing really fancy, just to be able to write all kinds of notations.
I would prefer a free one for Windows.
MuseScore is an easy-to-use sheet music writer. The best thing is that you can export to PDF, PNG, WAV and other formats!
It works on Mac, Windows and Linux.
Here is a C Major scale I made for the piano:
If you look at the top of the image, you can see all the different notes you can use (and this is in the Piano music mode).
One more thing: It's free!!
It quite depends on the complexity of whatever you're writing and whether you want WYSIWYG and things like MIDI keyboard input. In my experience, people who create music require both, those who are just typesetting are usually happy with the computer keyboard option - it tends to be faster than point 'n click.
AFAIK, the most popular commercial solution is Finale (both WYSIWYG and MIDI options), but it's really expensive.
My personal favourite is the Lilypond scripting language (think LaTeX for sheet music) with the Frescobaldi GUI (cross platform). There's a bit of a learning curve, but the documentation is excellent and it's fantastically flexible. Also, it's open source/free.
Wikipedia has a Comparison of scorewriters where you can compare features of lots of such programmes. You can compare output formats, whether they support tab notation etc.
I tried a few, and I like Denemo (an interface to Lilypond, which user3326 mentioned):