You should apply through the IAESTE in your home country and make an account on the Exchange Platform.
You can find a detailed explanation of the documents in this guideline.
If you are not a citizen of the EEA, you need a residence permit or a visa with an additional notation for the ability to work. This visa can be requested at an Austrian embassy. For internships between 3 to 6 months, you need to apply for a visa type-D. You should make an appointment at the embassy as soon as possible. The information about other types of Visas (depending on the citizenship and length of stay) can be found here.
Note: these documents were required by the Austrian embassy in Tehran, Iran. Make sure to check the required documents with the embassy in your home country.
When applying for a visa type-D, the embassy requires the following documents:
As a paid employee in Austria, you are insured by the social security system, but you have to purchase insurance for the days before and after your internship. You can get an “International student health insurance Europe” from Swisscare.com or any insurance company of your choice.
The local committee (Boku) will help with arranging the accommodation and the confirmation of accommodation(UB). Please get in touch with them at incoming@boku.iaeste.at. You can also refer to this wiki page.