Whenever you feel ready. There’s no automatic progression in renshuu that spoon-feeds you material in a certain order. To start learning kanji, you need to create a schedule for kanji. The easiest way to do that is to use the Manage your schedules link on the dashboard.
Also, I recommend reading the section of the help titled I'm a beginner. What should I study first? Click on (?) in the menubar, then Top Questions, then scroll down.
Finally, welcome to renshuu. I hope you find the time you spend here rewarding.
