First of all, congratulations. You'll be notified of your mandate via letter. Further you'll be invited to the initiation sitting where positions such as presidency, co-presidency etc. will be decided, also via letter. You'll find more info below, each text corresponding to a certain position to which is finally added the wording of each academy.
As mandatee in the student council you'll need to represent the students of all the majors under your student council. This means, among other things, sending fitting students to each committe (commitee of setting the curriculum, appeal board etc.). Further you'll be representing your students at universities and helping them in their respective situation. Additionally you will do counseling which includes tutoring first time students to make it easier for them to start a major. Each student council has access to a budget which will be used accordingly.
You have a voting right at meetings, can create petitions and get a view of the work the chair does. Further you can let yourself be a candidate in a wide range of commitees. You won't have any explicit duties except for conscientiously and dutifully holding your mandate.
This is comparable to your work as inter-universital mandatee, except one plane higher. You'll be in contact with the ministery and other similar official postings as well as other academy representatives.
Your duties as per the HSG 2014 are found here.
(left in german, passage number enboldened)
Österreichische Hochschülerinnen- und Hochschülerschaft
§ 4. Aufgaben der Österreichischen Hochschülerinnen- und Hochschülerschaft (general duties of austrian academy representatives)
§ 11. Aufgaben der Bundesvertretung der Studierenden (inter-governmental duties)
Hochschülerinnen- und Hochschülerschaften
§ 12.Mitglieder und Aufgaben der Hochschülerinnen- und Hochschülerschaften (members and general duties of academy representatives)
§ 17. Aufgaben der Hochschulvertretungen der Hochschülerinnen- und Hochschülerschaften (general duties of academy representatives)
Studienvertretungen (student councils)
§ 20. Aufgaben der Studienvertretung der Hochschülerinnen- und Hochschülerschaften (duties of the student councils)