There are only five skills in m20. Applications of all of the other skills in d20 are collapsed into this 5-skill system. Skill checks are made by rolling a d20 and taking that result + the skill bonus, + the applicable Stat bonus, and comparing that to a GM set target (Difficulty Class, or DC.) Which skills and which stats? The GM will tell you, but in most cases it should be obvious. Climbing, for example, would be STR + Athletics. Dodging would be DEX + Athletics. (Those rolls would also be a stand-in for Fortitude and Reflex saving throws.) In a few instances, your bonus would be a level check rather than a skill--for example, a Will save in m20 would be done as a MIND + level.
The character's skill bonus is equal to his level bonus + any skill bonus granted by class or race. For example, a 4th level Human fighter would have an Athletics skill of 4 (because he's 4th level) + 1 to all skills as a human racial trait, and +3 as a Fighter class trait, or +8 total.
The five skills are Athletics, Communication, Knowledge, Subterfuge and Survival.
Characters begin with 60 + (MIND bonus x 10) Sanity points. Whenever a disturbing event is experienced by a character, they make a Will save (MIND + level, DC set by GM) to avoid SAN loss. If the character fails the roll (or sometimes, even if they succeed), Sanity is lost, based on the following criteria: 1d6 SAN loss = seeing a minor monster or reading a Mythos style book. 2d6 SAN loss = seeing a significant monster or learning a powerful ritual spell. 3d6 = meeting a huge tentacle monster face‐to‐face. Any time a character loses more than 5 points of Sanity, the character must make a Will save (DC 20) check or develop a neurosis/psychosis. Characters with 0 or lower SAN are permanently insane.
Equipment lists from d20 can be used if desired for added detail, but I prefer a more "generic" equipment list, certainly when it comes to weapons, armor and the like. Unwieldy equipment lists, and worrying about things like carrying capacity, etc. strike me more as an exercise in accounting than in gaming, and I'm not in favor of them.
In this "generic" equipment list mileu, the following is the complete weapon list:
Unarmed - cost is free, damage is 1d3
Light (daggers, rapiers, etc.) - cost is 5 gp, damage is 1d6
Medium (swords, axes, etc.) - cost is 15 gp, damage is 1d8
Heavy (greatswords, two-handed ax, etc.) - cost is 20 gp, damage is 1d10. Cannot use shield with this size weapon.
Thrown (daggers, tomahawk, etc.) - cost is 1 gp, damage is 1d4. Range is 30 ft (no incremenets; it's either in range or not.)
Ranged (Bow and arrows, crossbow, etc.) - cost is 40 gp, damage is 1d8. Range is 100 ft. Assume unlimited ammunition (as per most action movies!)
Pistol - cost is 150 gp. Damage is 2d6, range is 50 ft. Must take an entire turn to reload after firing.
Rifle - cost is 200 gp. Damage is 2d8, range is 150 ft. Must take an entire turn to reload after firing.
Light (padded cloth, leather, etc.) - cost is 10 gp, Armor bonus to AC is +2.
Medium (chainmail or breastplate) - cost is 50 gp, Armor bonus to AC is +4
Heavy (full suit of plate armor) - cost is 250 gp, Armor bonus to AC is +6
Light shield (buckler or wooden shield) - cost is 10 gp, Armor bonus to AC is +1
Heavy shield (kite shield or fully metal shield) - cost is 15 gp, Armor bonus to AC is +2
Non combat equipment is not listed in m20, but should be references from the SRD.
Coming next: GMing and Combat!