I'll explain some of the races to you.
Orks: Savage, Warlike. They live mainly to fight.
Eldar: Rare, members of a dying Race
Space Marines: The Elite Fighting forces, made from the God Emperors Flesh
Chaos space Marines: Once space Marines who have turned there backs to the God-empoer, and worship the chaos gods
Imperial Guard: The largest and most dives re army. They are drawn from the hundereds of worlds in the hundreds.
Dark Eldar: Cruel and Evil, they live only to Torture.
Necrons: The Necrons are a mysterious race of skeletal warriors that have lain dormant in their stasis-tombs for millions of years. They are ancient beyond reckoning, pre-dating even the Eldar. Yet, they are beginning to awaken, for the galaxy is ripe for harvest.
Tau:On the Eastern Fringe of the galaxy, a new race has emerged to challenge the might of the Imperium of Man and the other established races in the 41st millennium. They are the Tau, a surprisingly advanced race that has organized itself around a single guiding principle known as "The Greater Good."
Tyrinds: Across the stars, a locust-like swarm of aliens attacked the Imperium of Man and the other species of the galaxy with ravenous ferocity. Known as Tyranids, this race of bio-mechanical monstrosities is driven by a single, narrow purpose: to feed. By ingesting the biomass of entire worlds, the Tyranids grow while absorbing genetic material so they can evolve into more-lethal killing machines.