Don’t worry, the rules are not that hard. I am 12 myself! Let me answer your questions.
In a combat move, you can move as many troops as you like from as many territories as you want. For example, if America took over the Kwangtung, the Japanese could make a joint attack from French Indochina and Manchuria. If there was two infantry and a tank in Manchuria, you could use all of them to mount the attack. If you do not have any troops in a territory, it is still under your control until an enemy takes it. You can mount completely seperate battles in the space of one turn as long as they don’t happen at the same time.
Now for the dice question. When you place your troops on the battle strip in the correct places, you can see a units attack number. You roll once for each unit, even if they have the same attacking or defending number. If you roll that unit’s number or less, you score a hit.
Lastly, I must give some fighter/bomber clarification. If you have planes on an island, and you see a ship or fleet coming towards, most likely for an amphibious assault, then you can bomb the ships IF the ships have finished their turn, and it is now yours. Remember, ships can move two sea zones per turn. I do not fully understand your second question. It does not matter whether enemy ships are in a sea zone friendly to you or not. You can send fighters and bombers over the ships both ways, as long as you have enough moves for the plane to get back to friendly land or sea territories. However, I would advise not to send bombers to attack ships that are in a sea zone right next to where the enemy has amassed AA guns.
That’s all. Don’t worry about asking simple questions. I did the same thing when I was first introduced to A&A.