1. Undefended Air Base Take
Srike the city hard and fast, take down the city ack guns first. Bombers for the center and cannon fighters for the outside is one strategy. Goons need to be close, C47s must be low and hopefully out of incoming fighter defense flight lanes. GV troops are a good option as they are harder to spot. Even if the base is undefended, it is a good idea to have some fighters ready to take out late uppers or fighters coming in from nearby airbases. When troops are released from the C-47s they must be at least 800 feet above the elevation of the city, otherwise they perish. Make sure when choosing the C-47 before you take off you load it with troops, nothing is more disappointing that flying over the city and releasing a load of vehicle or base supplies when everyone is expecting troops. The longer this takes, the smaller the chances to succeed as defenders will begin to show up and reduce your attacking numbers, they will come in with the advantage of altitude and energy. At best it then becomes a slugfest and protracted effort with reduced chances of success.
2. Defended Air Base Take
This is a harder task as you will find a great variety of conditions. If you plan the attack as "NOE" or Nap of the Earth, your incoming attack may avoid detection and give you the advantage of surprise. If you can get there unnoticed the best plan is to remove fighters and ground vehicles from the equation. Enemy planes aloft will no doubt cause some losses so plan enough fighters to compensate for that and be able to both deal with defenders both air and ground as well as remove the fighter/vehicle hangers. Heavily defended bases will need to have the bomber hangers taken out as well. Conditions during the early stages of the attack may well cause you to change from total destruction of the base to leaving some hangers up. If your fighters can "cap" the field, that is dominate it and prevent defenders from taking off, then leaving hangers up allows the base to be used immediately upon capture for it's own defense or launching subsequent missions. The city take remains the same, take out ack and enough buildings to turn the flag white. As an added note, the ack on the air field will need to be destroyed to allow a cap of the field. This capture takes more experience in the game as it is fluid and defenders may arrive from adjacent bases or from a carrier. Enemy ground spawns will need to be monitored as well for defenders. A defended base attack is very dynamic and often it is good to assign experienced piolots to co-ordinate various phases such as fighters, bombers and gvs if they are incorperated into your plan. As you gain experience, fly with missions to learn from those who lead them what works and what doesnt work. Co-ordinating a mission can be like herding cats as we are game players here, and not real military, so any number of alternate plans will be suggested en-route and likely confuse the issue if you don't keep the troops focused on the original plan. Having said that, if your plan has flaws, be open to recognizing that and adapting as you ingress. Too many changes or indecision however will affect the confidence of those with you, so once close to the target, maintain a plan and execute it. As it unfolds, make changes if needed. FUBAR is always flying with you and will exert his will at the worst moments, it happens, get over it and on to the next mission. This is only a very partial answer and many other variations exist.