The possible races for your character will depend on the supplements
that are in use by your group.
If you just have this rulebook, you will probably want to play a human, although any other creature in the Race Descriptions chapter is permissible (if you were playing a game based on Watership Down, your characters would all be rabbits, and there might be a few canine or feline NPCs).
Like this rulebook, most supplements contain a chapter detailing the various creatures that may be encountered. If you have Galactic Horizons, you or your GM may even create your own races according to the guidelines provided. Your character may be a member of any of these races.
Keep in mind the campaign you will be playing in, as some races are ineffective certain campaigns. (The aforementioned rabbits might not fit well into the world of espionage.)
The race descriptions include a list of average attributes, height, weight, and anything else required to create a character, as well as the various movement values, described more fully in the section on Movement in Other Rules. They also include any important aspects of the race, such as social order, combat techniques, and so on.
They also provide a list of all hit locations which apply to a character from that race, give the calculations to determine the maximum number of hit points your character will have in each of the hit locations, and tell you what the armour class for an unarmoured member of that race is. See the introduction to the Race Descriptions chapter, and the chapter on Combat, for more detail on the significance of these numbers.