In the airspaces you do have satellites and interceptors however, you'll be able to detect any intrusion and send airships to deal with them accordingly.
Depending on their launch station, they will have a short time to reach the UFO..
And engage in combat with it. There are a variety of different stats in these kinds of battles and you will need to seriously re-haul your air-fighting capabilities If you hope to be able to deal with the bigger UFO's. You do this by both researching and building said ships, which you can outfit with several kinds of weapons.
An interceptor have a short time to either get shot down itself, or shoot down the enemy UFO in time before the contact is lost. The timer at the top showcases this. But in the beginning of the game, your standard interceptors will do just fine.
After shooting down that UFO, my team had the possibility to fly over onto the crash-site, wipe out any remaining aliens and scavage the rest of the ship for research and profits alike.
Even after short periods of time, the UFO's, given enough luck on their end, will kick your interceptors arse If not upgraded soon and letting them get away could ruin your satellites, and in turn, your income.
Building a solid and throughly thought-through base with adjacent facilities that are identical to grant themselves bonuses in efficiency is absolute key. Here you can see 3 workshops on top right and right under that, three generators that are both boosted by the natural underground Steam, making them special Thermo-Nuclear Generators.
Keeping watch on your research is very important. Fluking it up and forgetting to assign a project can cost you the entire game. Keep an eye on this in the incoming Events tab.
We have spear-headed the research on bodyarmor for our soldiers, namely the Carapace armor that grants our soldiers huge amounts of health-boosts.
Complementing the Carapace armor is the Nano-Fiber vest. It is an item to be equipped in the Misc spot and It's almost an essential item to be given to soldiers that get up close-and-personal with the aliens, namely the Assault.
That is however, the limit of the general strategies I can teach you for X-Com: Enemy Unknown. The game is all about resource-management, planning, strategizing and sometimes, just getting a bit lucky.
Best of luck to you and good hunting!