Student or Learner
May I ask when you are free (for?) today?
I would like to text my teacher whether he is available/free so that I can meet/see him. Is that the correct question?
The "for" is definitely not necessary.
May I ask when you are free today?
When are you free today?
Are you free today at all?
Are you free at all today?
Do you have any free time today?
I would follow any one of those with "I'd like to meet up".
Remember - if you don't use correct capitalisation, punctuation and spacing, anything you write will be incorrect.