Well, to start off on Mizahar you create a character sheet first. There are plenty of resources around to create this sheet, including completely ready made character sheets (called templates) for every city in Mizahar. You can find all sorts of extra information on this in the
character sheet section of the forum.
Once your character sheet is finished you post it in the character sheet review 3.0 thread. Once you have done that you're ready to start writing. If there is anything wrong with your character sheet, you will be contacted by one of the Storytellers. If this happens, you're not allowed to write anything IC( In Character) anymore until your character sheet has been fixed.
To start your character of IC, you simply post in the correct section of the forum. For example, my character started in Avanthal so I went to the Avanthal part of the forum, posted in the character registry for this season, posted in the Gates thread where all arriving characters have to post first and then I went and started my character by creating a new thread about my arrival and meeting new people. Depending on where you start you will see similar threads where you need to post first before being allowed to start writing in a certain city.
All of this information and more can be found in the starting guide in the Lore section of the site. If you have any more questions, don't hesitate to post them here or PM me and I will answer them as best as I can.