How do you get seeds in xbox360 version?

That depends on whether you mean "seeds" the item or "seeds" the world generation code.

The item can be obtained in three ways.
1 - Tall grass has a chance of dropping seeds when destroyed. A bucket of water can destroy large amounts of tall grass at once.
2 - Harvest (destroy) wheat. Wheat drops 0-2 seeds when fully grown. You can find wheat in most NPC villages.
3 - Switch to creative and simply get seeds that way.

The world generation code for one of your maps can be seen at the level load screen. You can randomly enter letters, numbers, and symbols to see what happens, or you can find lists online of various seeds. Be careful, you need to find seeds for the most current Xbox version. PC seeds (of the same build) no longer work on the Xbox due to 4J changing the world generation.