How do I start?
The very first thing you need to do is create a character. You can play up to 3 characters in BattleMaster, assumed to be brothers or other relatives from the same family or clan. There should be a link called Create a new character on the user page (which is the first page you see after login).
How do I create a character?
On the page you get to when clicking the Create a new character link, you need to input a first name for the character, choose a gender, and the type of unit you want to start out with. All characters begin the game with a bit of gold and a small unit of your chosen type. If you are unsure what to choose, take infantry, which is useful for all kinds of activities.
You also need to select a realm to start in, which is the most difficult part of creating a new character. You can find information about realms in the "Game" part of the main navigation, and on the character creation page there is a link to a page with descriptions of each realm, written by its ruler. The game will also suggest three realms, and mention three other realms that it thinks you should not choose.
Once in the game, what should I do?
Most importantly, you should take it easy. BattleMaster is not a game where you will do a lot in a day, it is meant to be seen on a more long-term perspective.
When you enter the game, a message is automatically posted to your realm members that you have arrived. If there are any mentors looking for students, one of them will probably contact you, but this can take several hours. So spend some time to familiarize yourself with the options you have, take a look at the information available about your realm, your character, etc. Take a look at the status page where it will tell you when the next turn (either sunrise or sunset) will happen. If you did not appear in a city, look at the map and maybe travel towards the nearest friendly city. Also make sure to read the realm bulletins, they should have something you can do in it.
Then log off and wait for the turn. When you log in next time, you might have a mentor offering you to teach the ropes, and some interesting events to get you started.