The major scale is made up of only 8 notes across 12 keys. Pick any note, then count each key and follow this pattern: R-2-2-1-2-2-2-1(r), repeat all the way up.
Follow that scale pattern until you're familiar with all 12 major scales. It'll make it super easy to figure out the next part....Chords.
You can now figure out a song by looking at any lead sheet or chord chart- for example, try playing these chords: C- Am (a minor)- F- G. Happy music making!