Chords, scales, notes, shapes, frets, modes, keys, bends, vibrato, harmonics, hammer-ons, pull-offs, legato, trills, slides, double-stops...
The first things to learn are the 5 chord shapes for accompaniment and the chromatic scale for melodies.
The 5 basic chords are E, A, G, C and D. An easy way to remember this is the word CAGED. Your fingers must be trained to jump right into the necessary contortion for each chord from any of the other chords. "Running Down A Dream" by Tom Petty is a perfect song for practicing all 5. Once you can easily play any of the 5 chords, the next chords to learn are A minor, E minor and D minor. Once you know these you will be able to play a majority of the songs out there!
The chromatic scale is simply every single note on the fretboard. Each of the 4 fingers in the left hand is used on a successive fret, so all notes are included.
Here is a simple practice technique you can use to strengthen your individual fingers and as a warmup before each playing session: Use each one of your fingers on successive frets on the same string, putting them down and picking them up while plucking the string. Make sure that you are pressing down hard enough on the string so the tone sounds cleanly, but not so hard that it gets pulled out of tune. If you hear buzzing, you are not pressing hard enough. If you feel fatigue you are pressing too hard.
Strumming, slapping, popping, tapping, palm muting, picking, fingerpicking, chicken pickin', hybrid picking, sweep picking, economy picking...
Choose a pick with solid structural integrity and a good grip. Place your thumb on the front face and curl your index finger over the back. Point the narrow end at the strings and brush it over them with a brisk sweeping motion from the wrist. Brush them again on the way back - this way is called an "up" pick.
The right hand creates the sound that the instrument gives off with different attacks. Train your hand to hold the pick firmly with your fingers while keeping your wrist muscles relaxed. Swipe the pick over all 6 strings in one quick motion and feel the pick bend. Listen to the raking sound as it clicks against each string quickly in succession. Press the blade of your palm along the strings in the area of the bridge to mute the strings and play more percussively. Gently drag the edge of your thumb along the strings for soft, gentle chords. Pick on individual strings in sequence with the tips of your fingers. Reach underneath the string with your fingers and pull it from the fretboard, then release it and let it snap against the frets. Flick the strings with your fingernails. Pinch 2 strings together.
Buy a metronome and practice with it every time!