The army lists are in section 6 of the rulebook. They detail the composition of formations and many points each costs. They are used for "tournament games". Essentially, you and your opponent agree to a point cost, make your armies, and setup the game as per section 6.
There's also the option of scenario play, where you can pit whatever forces you like against each other. Although those can be hard to balance.
Head over to TacCom if you have more Epic questions:
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It's the best Epic resource online.