Print maps
Any travel guide to Vienna comes with some form of map inside. They're fine if you're going to stay in the very center of town, but not much use if you'll be exploring the city's outlying sights (like the Zentralfriedhof, for example.)
However, before arriving in Vienna, don't bother investing in buying a "proper" map.
Before you leave, you have the online maps to take care of any travel planning you may want to do.
When you get here, you have every bookshop and souvenir stand selling maps. Plus the tourist office and many hotels have a perfectly-adequate free (sponsored) city map for you to take with you.
This free map folds down to about 3 inches by 3 inches and has a picture of St.Stephens Cathedral on the front: it's a bit flimsy, but will last for your stay.
On one side it has a detailed map of almost the entire city, including bus, tram and subway transport routes. On the other, it has various "sponsor messages", plus a subway map and a more detailed map of the city center with the major tourist attractions marked.
If you don't have one pressed into your hands at reception, pick one up from the Tourist Information Office, or ask the receptionist if they have a "Stadtplan" (pronounced schtat-plan).
One reason to consider buying a more formal street map is that the free version has no street index. One well-recommended purchase is the ADAC City Plan. It's a small fold-up map, but covers the city and its environs in detail, includes public transport routes and has that useful street index. It costs about €6.50.
You might also spend €1.50 on a map from the public transport authority, available from their ticket offices, mainline railway stations and wherever they have manned information offices (the subway stations Karlsplatz, Stephansplatz, Westbahnhof, Praterstern, Philadelphiabrücke, Landstraße, Volkstheater, Foridsdorf, Erdberg and Schottentor).
It's the Verkehrslinienplan, a large folding map of the city with all the public transport routes marked on it. More usefully, each stop or station on every route is also labelled clearly with its name.