Before you can begin, you'll need a piece of software to do all of your musical work in. Somewhere you can have multiple tracks of audio or sequenced MIDI notes, where you can apply effects and have everything musically in time. This is a Digital Audio Workstation.
Garageband (Comes with MacOS)
Pro Tools First
Studio One Prime
Trial version available
Less than $100
Mixcraft Home Studio
Mixcraft Recording Studio
Ableton Live Intro
FL Studio Fruity
Studio One Artist
Less than $300
Waveform Pro Basic
Mixcraft Pro Studio
Waveform Pro Standard
FL Studio Producer Edition
Logic Pro X
FL Studio Signature
More than $300
Studio One Professional
Ableton Live Standard
Waveform Pro Extreme
Ableton Live Suite
FL Studio All Plugins
Pro Tools Ultimate
In the DAW, these are your tools to create and modify sound.
VST instruments are plugins you can load up and sequence notes with.
Sforzando (sfz/soundfont player)
T-Force Alpha Plus
Orion Sound Module
Virtual Playing Orchestra
Magical 8bit plugin
SocaLabs 8-bit Treats
EastWest Composer Cloud
Here are some presets to use with the instruments, as well as sample packs for sample players that work in pretty much the same way.
Audio effects! Also useful if you're just mixing covers!
Sample packs: like an UTAU voicebank but for instruments. Usually for drum sounds!
And some other stuff you may find useful, like websites hosting many different kinds of plugins you can also check out.
How do you decide what notes to use? What sounds go together? How long should each part of the song be? That's what music theory is for, so dive in and find your inspiration.
Chord Progression Generator
Websites for guitar chords
http://www.guitar-chords.org.uk/ - Reference
https://jguitar.com/chordname - Enter strings/frets and get chord name
https://www.jamplay.com/tools/guitar-chords - Reference
http://www.all-guitar-chords.com/ - Reference
Sometimes it helps to have people show you the ropes
Sometimes it helps to just get inspired!
Lyrics, you can't have singing without em.
Written guides for songwriting and mixing
Lots of other resources to help you with music!