Creative Suite from Adobe for starters, if you're not doing any design you could probably get by with just Acrobat Professional. Most customers at a small shop like to bring in stuff like Microsoft Office and they'll have the latest version so you'll need that. I also recommend some sort of imposition software, either Quite Imposing or Debenu Aerialist work fine for SRA3 digital stuff. You'll also want a copy of Enfocus Pitstop Professional. I would consider these essential to running a working shop. If you start doing design or touching up other's design you'll want to get Creative Suite from Adobe with Indesign, Illustrator, and Photoshop (if you can I would recommend buying CS6 rather than moving to the Creative Cloud versions...personal preference). Other nice software to have are things like an estimating package, accounting software like Quickbooks, OCR software like Omnipage Professional, mailing software like Satori Bulk Mailer or Melissa Data, VDP software such as Fusion Pro, XM Pie, Pageflex and the like. I've also found little pieces like PDF protection removal, timeclock software, and DVD/CD burning software are very handy.
I want to second Jotterpinky's recommendations. You'll also want a server, NAS box or cloud storage for storing your customer art files. I'm all about the versatility, especially when starting up, but you'll want some type of dedicated system for storing files. If you have those art files on a workstation and then have problems with it, you could lose those art files. And some backup software with a disaster recovery plan.
If you're just starting out, the Creative Suite will help you immensely. For those customers with Microsoft software, ask for a PDF. You can fix many PDF problems in Acrobat Pro, or you can get Pitstop to cover all bases. You can impose in InDesign for starters.
Make sure your digital printer comes with a RIP such as Fiery. It lets you tweak a lot of things, such as registration being off when printing.