How can I be national team manager easier?

You just need experience, if you do well over a period of time you will get it.

You can be a national team manager at the start by clicking on "choose from list" when you pick your club, browse through all the teams and players until you find a player who is the nationality of the team you want to manage and click on where it says his nationality then it will come up with the national team screen click on "take control" and thats all you need to do.
For example if you want to be manager of England you would click on "choose from list" then find a team with a player who is english, say man utd, then click on ferdinand or any other english player and click on where it says "english" under his name.

I was AC Milan and I had to wait until my Reputation was Superb to get the Italian Managers Job. If you Apply at Very Good you probably won't get it.

I got the England Managers job when my reputation was Good