The Iliad is not the whole war
It seizes one burning moment: Achilles' anger, the fragility of chiefs, Hector's death and the strange dignity the poem grants even to enemies.
The Odyssey is not a simple sequel
It begins when glory should have been enough. Odysseus has won, but he still has to recover his name, his bed, his son, his kingdom and the peace war has almost made impossible.
Read without confusing the poems
To prepare for the film without mixing the poems together, start with the reading order, then check why The Iliad does not directly tell the Trojan Horse.