I'm no Grand Pubah of A&AM, but I'll take a shot at these...
-How many units in a hex, infantry and vehicles, infantry with vehicles?
You can have 2 of your own in a hex, but no more than one vehicle. So you could have 2 Garands, or 1 garand and 1 Sherman, but not two Shermans. The grand total of units including enemy units is four units, but again, no more than one vehicle. So if you had that Garand and Sherman in a hex, the enemy could move two SS-Panzergrenadiers into the hex, but not one of them and a King Tiger.
-Do units have Line of Sight through friendly units, and if so can they fire through them at the enemy?
-Does a hill give your units the capability of shooting over friendly units not on a hill?
See above question. Hills give cover, block line of sight and often funnel AFVs who have a hard time getting through them, but other than that they don't do much (except for some special abilities, that function in hills).
-How does charging an enemy hex work?
You move into the hex and provoke defensive fire. Soldiers don't provoke defensive fire from vehicles.
-If a unit dies from defensive fire, can it still fire back?
Yes, defensive fire only causes disruption (which means that the enemy who gets shot up doesn't make it into the hex he was going for). The disruption is immediate in that you don't wait for the casualty phase to flip it over. This means that the guy firing back, the whole who provoked the DF, has a harder time shooting back now (as per disruption).