WARNING: Climbing is considered a dangerous activity. No written material can substitute proper teaching. The following is a just a list of topics, but you should seek a professional instructor.
There are quite a few important things, assuming you mean sport-leading (trad climbing involves more):
- Quickdraws: understand about gate directions, z-clipping, reverse-clipping, rope-travel, and rope-drag. Practice clipping techniques.
- Belay device: auto-locking (grigri) vs. standard friction device. Rope feeding technique, assessing slack in the system, when to feed out more slack and when to "listen" to the rope (e.g. be ready to catch a fall.)
- Lead belaying: feeding out rope, amount of rope to feed (slack), how to do it (see belay device,) belayer position and stance (safe location, ready to be yanked/jerked by sudden tension/pull due to a big fall), catching falls (work on soft/dynamic catch, possibly jumping), never letting go of the brake hand, spotting until first bolt is clipped.
- Communication: "Clipping!", "Clipped!", "Falling!" in addition to standard climbing phrases.
- Lead Climbing: clipping bolts (at waist height unless there's a good reason to do otherwise, clipping rope without putting finger through gate (lots of practice), quickdraw orientation (biner gate facing away from rock,) rope management (with respect to feet in particular,) falling (being ready to land, watch your ankles, wrists, head, etc.)
- Anchors: full top-rope anchor vs. double-quickdraw vs. cordellete, in addition: standard anchor stuff (SERENE, understanding direction and distribution of load, etc,) cleaning anchors (safely, personal anchor system, how to transition from belay to PAS and back safely)
- Multi-pitching, belaying from the top (using grigri, using auto-blocking (reverso, atc guide)), belaying from an anchor (as a second), cleaning as a second
- Understand dynamic ropes, fall factors, stretch, etc.
- Explain foam helmets (protection in fall, not just from rockfall from above), belay gloves
- Knots: clove/full hitch (for use as personal safety using the rope attached to your harness), munter/half hitch (emergency belay, say, if you dropped your belay device...), right-hand bowline (useful for anchoring trees, for example), rethreaded bowline (safe for ring-loading, non-capsizing), understanding behaviour of knots under pressure: ring-loading, capsizing, blocking knots, passing knots (to pull knots through rings/biners), etc.
Should already know: rappelling, setting and cleaning anchors, using a prussik as third-hand belay in rappel, ascending on a prussik,
Personal note:, I can forgive new climbers most things, except for the severe lack of practice in belaying. This is a serious issue, where a lot of self-taught and non-indoor climbing climbers often lack until very far in their climbing careers, due to a reasonable unwillingness to practice on shoddy outdoor anchors and on their own ropes. It's absolutely worthwhile to go to an indoor gym every now and then and spend a few hours practising leading, falling, and catching falls.