This is a technical violation, and probation understands that sometimes people who use drug relapse. Probation also knows that you are brand new to the program and that marijuana can last in your system a long time. You can call your PO and tell him that you probably will fail the test. In my experience, while probation won't be thrilled about the positive, this first time your PO will probably cut you a little slack.
Probation has a whole arsenal of possible sanctions available to them, ranging from a wrist slap (more intensive supervision, more reporting requirements) on the one extreme to house arrest or revoking probation outright on the other.
What they will do in a particular case will depend on many factors, such as a defendant's background and history and the facts and circumstances of the underlying case, how well he's been getting along with your probation officer, whether he's been in good over all compliance with the terms of his probation before the violation or whether he's given them trouble before. You get the picture.
It will all come down to whether probation thinks that working with you will ultimately turn you into a successful probationer and a productive citizen or whether they think you never will get the message and will want to wash their hands of you.
In my experience, they will likely just increase your reporting and your testing schedule. You haven't had much of a chance to get the benefit of probation. But if they do decide to violate you, you will be entitled to a hearing on the violation.
You should have a lawyer at that hearing so that in the unlikely event that probation and the DA are asking for you to be resentenced to jail, someone can make a good argument for drug treatment or house arrest.