If you want to know the basics on how to create music, learning basic music theory, if you haven't already, will help tremendously. I am going to write this as if you do not know anything about composition to make it simpler for myself.
1) Chord Progressions - When starting a song, I start with one instrument and make chords then stick to them. If you don't have a solid chord progression, the rest of the song won't sound right. If you have a hard time making up a progression, you can easily find common progressions on various websites.
2) Melody - The most memorable part of a song is the melody, or at least almost always.
To create a melody, do any of the following:
-Hum to yourself until you've found something, and place the notes accordingly.
-If you know how to play an instrument and have one with you, do something similar to a solo on top of the chord progression, taking note if anything is of interest.
-If you know the notes in the key signature, put the first note on the first note of the key and press the spacebar over the other notes and see which ones sound best next.
3) Percussion - I have just recently learned how to correctly do percussion, so I will only say the very basics: Bass Drum on the 1st, 2nd, and 4th beat, Snare Drum on the 3rd. (One beat being a quarter note's length.) This is as basic as it gets, but it works.
4) Bass - I honestly do not know the best way of writing for the bass. An easy way of doing this is to place them on each quarter note on one note for a chord, and change it for the next.
This is all you need for a basic song, but the more the merrier when you get used to it.
NOTE: Don't actually do this, It won't help at all.