How to enter the seed in minecraft?

Click create new world
More World options
Will look like this
The top bit says 'Seed for the World Generator', type the seed you want in there
Then Create new world and the seed will show up.

Note some older seeds won't work with newer patches due to the worlds generating differently.

Go to the bottom right corner, click create new world. Then go to more world options under game mode and there's a text box for the seed. Enter your seed and the world will spawn. There are also many other options on that screen, so check those out.