... Well clan basic training basically (pun intended) is HELL. You're born, and you're trained from the moment your eyes open, they imprint a warrior life style as young as possible and you develop quickly, before a clanner knows how shave he will know how to break a mans neck.
Clanners are brought up in Sibco's, these are... Err, "batches" of True Births, they run, they fight, they shoot and they take an extremity of tests, they are encouraged to duel each other, kill the weakest, and weed out the strong and leave only the strongest left standing, living. They at a young age learn to dispel love and such, sex is considered nothing but a pass time to drain a soldier of attachment and emotion towards anyone of the opposite, or same, sex.
They begin to handle mechs after they are judged worthy enough soldiers, most Mech Warriors of the clans would be the most elite ground infantry, trained to have incredible reflex's, combat situational awareness, and a toughness that is literally BEATEN into them by their trainers. Their bodies are hardened from days comprising of drills, exercises, running, shooting. They often don't get fed to build will and character and see who simply can't make it (often these will get bumped down to clan infantry, or become one of the other caste's).
When few, and I mean a very few of these sibco remain, they are given their first mech, they train constantly in it, marksmanship, proper clan etiquette , tactics and are taught how to command even if they are going to remain nothing but low ranked MechWarriors. In this time, they still hone their bodies, are trained even harder by their new instructors, and then finally when done they enter the Trial By Fire. This trial isn't in all clans but most, where the newly trained warrior is paired against an aging warrior that is simply too old or disgraced, these two will duel, often resulting in death a sort of Pheonix like rising, one warrior dies so a new can begin, this will be the end of the hellish training of physical torture, pain, emotional breaking, and conditioning..
This is the point where a clan MechWarrior is graduated, there is no comparison to their training in modern history.. The Trial by Fire can also be taken in battle against an enemy if the fledgling proves himself in combat. Those warriors who are rejected often are bumped to infantry, or just killed.