I agree with Peachey, you should use the contact teacher button. This will help us to find out a lot of information, such as: how often would you like to learn, which language, what is your goal and if there is anything else you would like to do. In the part "Is there anything else you would like to ask" you can ask about the certain day and time you'd like to have a lesson.

There's a "Contact Teacher" button on each teacher's profile, so you can use that to ask if they are free at a certain time. The message box is below a whole lot of tick-boxes, so just skip those and type away. Speaking as a teacher, I always appreciate it if a potential student takes the time to message me about what they want to do,