Olivia,
If you decide to go directly to Gimmelwald (which is certainly possible), simply buy your ticket to Gimmelwald at the Zürich Airport station. When buying your ticket, specify the final destination as "Gimmelwald Schilthornbahn". It will include all modes of transport. You can determine prices by checking the sbb.ch website.
There are two routes to reach Gimmelwald, and this is the route I'd suggest.....
- Train from Zürich airport to Interlaken Ost
- Transfer to smaller Berner Oberland train for ~20 minute ride to Lauterbrunnen
- Transfer to Post Bus for ~20 minute ride to Stechelberg (it's bright yellow, you can't miss it - the Bus will be timed to connect with the train)
- Cable car to Gimmelwald (~10 minutes)
As mentioned previously, you may find it beneficial to use one of the Swiss passes, regional passes or half fare cards. That will depend to some extent on where you plan on exploring in that area, and whether you'll be using a lot of the mountain lifts.
For travel in August 2014, you may have some difficulties finding accommodations. You may have to consider staying in Mürren, Lauterbrunnen or Wengen (all of them would also be great). Lauterbrunnen is the most central to use for a "home base" in that area, as it's a good transportation hub to both sides of the valley.