Decide how the aliens will invade. Typically, alien invasions take three forms: infiltration (with aliens assuming human form), alien raids to strip away our resources or as staging grounds for a larger invasion and full-scale alien occupation. The aliens don't have to be evil. In some cases they could just be monitoring other aspects of the solar system (as in Arthur C. Clarke's "Rendezvous with Rama" where the alien probe never seems to notice human activity).
Design your aliens. Like it or not, your alien characters, not your heroes, will draw readers to your book. The possibilities are endless: aliens can resemble or assume human form; they can be bugs or lizards, green or gray anthropoids, evolved beings, a military species or even the creatures who initiated human civilization.
Develop your human characters. Your human characters should be as interesting as the aliens, and we should want them to succeed. Typical characters include a civilian hero and his family caught off guard by the invasion who must rally others to the cause, his allies (each of whom can contribute some unique skill to the cause), the skeptics who don't believe there are aliens or collaborators who help the aliens. Be willing to sacrifice any character (except your hero) to the aliens.
Plan the resistance. Earth doesn't have to win, but they should put up a good fight. In the case of alien invasion, military might is never so effective as cooperation and problem-solving. Usually, aliens are defeated by a weakness that humans don't suffer (the common cold or a computer virus, electromagnetic interference, even vinegar and/or water).
Make an outline to work out potential problem areas before you even begin writing. Novel outlines should break down into three parts: the setup, in which the heroes encounter the aliens; the first wave of resistance, which utterly fails, and the climax in which the aliens are finally rousted (or insidiously take over everything).
Write. Don't worry about making mistakes or following your outline exactly; go where the story takes you. Add surprises and plot twists (the aliens have their own resistance, Earth's army is infiltrated by aliens, the hero's love interest is an alien in disguise). Add a cliffhanger to every chapter to make your novel difficult to put down.