Well, first off, exploration costs more for each acre of land that you have.
So, it's better to explore in big chunks, not just a little at a time.
If you are a beginner, it is wise to get about 2000 acres, build a decent army, build 50 forts and 50 training grounds, and 1900 mines. (Or you can build a few extra forts if you're concerned about your defense.
Log in after a few days, you should have about 10 million in gold.
Use it on further strengthening your army, but not exploring.
Log in again and when you see your total hit 20 million in gold, start exploring.
Do the max amount of exploration.
You're kingdom is now a decent size, not to mention making a lot of money.
You are now ready to focus on your hero, or start attacking other enemies.
You are now ready to win.

(Added by an experienced player) This is a slow way to go. Attack attack attack. There are a lot of small kingdoms out there that have been abandoned by their owners who don't care, in some cases they can have over 30 million in gold and no one defending them you just have to look.


(Added by another experienced player)
That strat never works, especially now that warbook has been fixed so you can only buy a little bit of land at a time even for beginners.
Best strat is buy land time 20,000acres. Till then do not attack people because the cost of your army is more than your land cost till then. At all times, your DPA(Defense Per Acre) should be about 100. so for example, if you have 20,000acres of land, you should have 2,000,000Defence at the least or else you will lose a lot of land to other people. After you reach 20,000acres, start attacking people a little below you till you reach 80,000acres. This strat works for auto, magician, general, slayer, warmonger, and sometimes Moguls.