Both Austria and Slovenia require Vignettes for driving on the Autobahns / avtocesta. These are toll stickers which you fix on your windscreen. Instead of the toll being based on distance, it is based on the number of days, it is therefore a lot cheaper to administer. Vignettes are available for various periods (week, month, year, but ech country is different).
You can usually buy them at the border, this is the normal way for for tourists to buy them. Otherwise garages or post offices usually sell them.
Click here for a photo of an Austrian 10-day vignette.
If you are getting a hire car, you can ask when you pick up the car. Hire cars usually already have annual vignettes for the country.
If it is a hire car, I hope you are bringing it back to Austria. One-way car hire from one country to another is very expensive.