I've talked about training before (make an "acquisition roll" or Intelligence check after spending appropriate time and money, success means the "skill" is acquired.) I should explain how I see this in terms of weapon proficiencies and which weapons are usable by a class.
An ordinary human -- the so-called "0-level characters" -- has a basic ability to fight with punches, kicks, thrown rocks, clubs and knives or daggers. In Target20 terms, d20 + AC => 20 hits with these basic weapons; in my alternative combat mechanic, d20 + (2 x armor rating) > 16 is a hit for a 0-level character. All of these characters also get a 1-point benefit from the passive protection of a shield.
In addition, if a character's background includes use of a weapon, that weapon has the same chance to hit. Hunters can use a bow as well as a dagger or fist; shepherds can use a sling at no penalty. All classes except Fighter add their attack bonus (= hit dice, for me) to attacks with any of these weapons. Untrained weapon attacks do not add this bonus, and if a character has never seen or used a weapon or combat maneuver before, there is an additional 1-point unfamiliarity penalty.
When using an unfamiliar weapon or maneuver, triple the damage roll and compare it to Intelligence; if the result is lower, the character is now familiar with that weapon and can use it without the 1-point penalty. Training as described above not only eliminates the unfamiliarity penalty, but also lets the character use their HD or level attack bonus.
Fighters, on the other hand, can use any weapon or combat maneuver. If they haven't seen or used a weapon/maneuver before, they do get the 1-point unfamiliarity penalty, but still get their attack bonus -- so a 1st level fighter can try anything in combat and be as likely to succeed as a trained 0-level man-at-arms; once that fighter becomes familiar with that weapon or attack, the +1 HD bonus kicks in. Fighters never need weapon training: that's for peasant levies, pages, squires of a knight, or those otherwise unskilled in combat.