Travel from Vienna to Hallstat

Are you using: http://www.oebb.at/
This is the official Austrian Railways website. If you look in the top right there is a button "English". Click on it.
Enter WIEN (German for Vienna) to HALLSTATT (note spelling), and pick a date and time.
I tried a random date in December. It shows 2 trains every hour, journey time approx 4 hours, prices varying from €19 to €49.50 per person, second class, each way, depending on which train you choose.

Thank you so much, that helped a lot! My friend and I were able to get some decently cheap tickets round trip!

Your original post said "I", but if there are two of you, and you don't mind spending 6 hours ±½ hr, you can use regional trains and a 35€ Einfach-Raus-Ticket (for both of you). That's an all-day (after 9 AM workdays, any time on weekends, until 3 AM the following day) pass for unlimited on/off travel on regional trains. You need at least two people (up to 5); a single traveler cannot use the Einfach Raus ticket.

With an 11 - 13 hr round trip, you probably don't want to do it in one day, so it will take two Einfach-Raus-Tickets to make the round trip with an overnight stay.

As the others have indicated, due to time for transportation each way from Vienna you may want to spend at least one night in Hallstatt. Regarding the rail journey, one of the easiest trips appears to be a departure from Wien Westbahnhof at 07:48, arriving Hallstatt at 11:24 (one change from OBB to REX trains at Attnang-Pucheim, which is standard on that route). When you arrive at the small (unmanned) station in Hallstatt you'll need to walk down a small path to the boat dock, and then take a short ride on Stefanie across the lake (~€2.50 PP each way). Have your camera ready, as the views are spectacular!

Thank you all for the suggestions! We will look into spending the night up there if the deals are good enough! Can't wait

I would go to Halstatt if convenient but I for one was a bit disappointed with it. It's pretty, no doubt, but it is also overrun with tourists and that tends to ruin even the best European villages. If you go, don't get your hopes up too high.

Hallstatt is not really a day trip from Vienna. You are wise to spend the night. There are a couple of nice campgrounds there, if you are on a budget.

When I was in Hallstatt, I actually stayed in Lahn, in a Privatzimmer. My single room was 22€/nt, including breakfast. Privatzimmer can be a great experience and a good value for someone on a budget, but are in a private home, where the owners often don't speak English, so some knowledge of German can be helpful. (Where I stayed, the older woman only spoke German, but one night an Asian couple stayed there, the next night Italians, none of whom spoke German, and they all managed.)

Lahn is south of Hallstatt, at the entrance to the tunnel that bypasses Hallstatt, about a 1 km walk to downtown Hallstatt along the lakeside pedestrian road. There is a bus (5 min) to Lahn from the manned train station in Obertraun, the next stop after the unmanned station across the lake from Hallstatt.

This page lists some Privatzimmer in Hallstatt/Lahn. I was originally going to stay in Haus Bergblick, but they had an opportunity to rent both double rooms so asked me if I would stay in a single in the mother-in-law's farm house across the road.