L carnitine and ALCAR have some positive effects with regards to the brain/ cognitive function and has some anti ageing effects but for the purpose of bodybuilding and fat loss its relatively worthless. It has benefits but not really for us.
Carnitine transfers long-chain fatty acids into the mitochondria where they may be oxidized to produce energy. The recommended dosage of L-carnitine is 2000-3000mg per day. You would want to take the L-carnitine with your pre-workout meal.
As great as that sounds, supplemental L-carnitine doesn't do a whole lot because your body can efficiently already create it's own. Supplementing with it certainly won't hurt but don't expect miracles. If you do decide to supplement with Carnitine, use the acetyl-l-carnitine version as it is the most bioavailable.