The first threat to your security upon making planetfall are aliens. Use your starting units to determine how much alien activity there is in your vicinity. If aliens are found to be a problem and often surround your city research Ecology so that you can build Ultrasonic Fences around your city, keeping the aliens at bay. In the mean time always protect your workers and colonies with some kind of a military unit, so that aliens don't randomly destroy them by walking onto their tile. If many nests are close-by you might want to go down the might tree and build some soldier units, using the aliens as a resource for you to take advantage of.