What is Reddit's opinion of NoteWorthy Composer?

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This is something you can easily do yourself, especially since you have the sheet music. Get a music notation program like MuseScore or Noteworthy Composer and plug your song in. You can save it as a midi file and then throw it in Synthesia.

Lots of notation software can do this. I'm not familiar with Garage Band but I don't believe it does notation, only editing of tracks. As well, after looking it up, Dent Du MIDI is definitely not the kind software you want - that just changes the format of MIDI files for use in other programs. The notation software I use most often is Sibelius, though its pretty expensive and would be a waste if you aren't big into music composition. However I believe it does have a free trial so you lose nothing by checking out its MIDI transcribing capabilities. My favorite less expensive notation software is Noteworthy Composer though I believe it is Windows only. I googled around and, though I haven't personally used it, MuseScore is free, open-source and available on all platforms; seems like it might be a good option for you.

These programs make importing from MIDI quite simple, it should be a top level option along the lines of "File > Import > MIDI" or something like that. However, you should keep in mind that it will not always make perfect sheet music. MIDI is its own separate format that notation software can only attempt to automatically translate into something readable. As you may know rhythm and timing can be defined pretty fluidly, and things like time signatures are not included in actual MIDI files, just the individual notes and their properties, so be prepared to fiddle around with timing options like minimum note length, time signature, etc in the importing options to get a more accurate transcription of the MIDI. I can try to explain why this is but I'd probably just confuse you - the point is, it's likely that the sheet music you come out with will be only somewhat legible and will require some human touchup to not be too confusing to sightread, but it should be mostly right and the pitches at least should be accurate.

I used to use NoteWorthy Composer before I started using Finale. It's free and I still see people recommend it even though I've not used it in nearly a decade. It will probably suit your needs an might be more user-friendly than professional options like Finale and Sibelius that have more features than you likely need.

They gimp it just enough so that it's not quite useable for the average project. I'd suggest something like MuseScore or Noteworthy Composer if you don't want to pickup a full copy of Finale or Sibelius.

The simplest, well-functioning program I know of to serve your needs is probably going to be NoteWorthy Composer:

http://www.noteworthysoftware.com/

This will use your computer's built-in MIDI synthesizer to produce basic MIDI audio, so you'll have access to all the 127+ musical instruments included as part of your default MIDI instrument set that's built into Windows. It also gives you some rudimentary editing functionality, so you can input and edit notes like a word processor in addition to allowing you to record your performance live from your MIDI keyboard.

If you want a more realistic sound or more advanced editing capabilities, you'll need to look into DAWs and samplers and sample libraries. Once you get a keyboard, all of this will be available to you, and you can easily turn your home computer into a full-blown recording studio and be the next Hans Zimmer or Daft Punk. But for now, if you want something basic, the program above should get you started. Read the FAQs on their website to learn how to get started using the program.