A couple of tips for newbies:
1) Ack must be down before you can drop troops. Even soft guns (ones that players can shoot) will stop a base capture, and EVEN if there's nobody in them!! If the field is far away from the town, you only have to deack the town.
2) On the map by each icon of a field there should be a yellow dot. This will tell you where the town is. Example, if there is a field icon and the dot is below the field, the town will be S of the field.
3) Vehicle fields do not have towns. Nor do most ports (exceptions noted below). You must shoot all ack at the V-field or Port, don't forget any! Then drop troops over the field, they will find the map room on their own.
4) Some maps have "Combo Ports" -- this is a port with a field AND a city all attached to the same base. These are hard because you have ack from the port, the field, the town, and if there is a fleet nearby, from that as well. These usually take a large or very coordinated effort to capture in the Main Arena. For these ports you must level the town as if it were an airfield, and drop troops over the town, not the port.
EDIT:
5) Troops are like bombs. You can salvo them. Type ".salvo 10" and you only have to fire them once. They have a set time that they take to jump, so it won't be instant. This just saves you from clicking the "Fire" button 10 times.