You of course also need a transformer to the 6021 (or was it included in the set?), otherwise you're ready to control your locos. Not the turnouts, though...

Hi John, and welcome
That was the easy answer.<img src=icon_smile.gif border=0 align=middle>
Of course, the Webmaster was right; you only need a 6021 to get started, and you'll have a great time going digital.

However, before rushing to get one, you may want to consider another option:

The most obvious alternative to the 6021 is the Uhlenbrock Intellibox (IB).
The IB contains 2 throttles, keyboard, memory and interface. Thus, it replaces not only the 6021 but also a whole array of other Märklin boxes, saving a lot of money in the long run.

With the IB you can also use all of your Märklin digital devices, they all plug into the IB.The IB software can also be updated (via a download from the web). The technical design of the IB is more advanced in several ways, which you may appreciate in the future (it's compatible with various other types of decoders and wiring systems). Most of us have preferred the IB rather than the 6021, and there are no problems whatsoever in terms of compatibility or quality.

In the near future, there may be still another option: the Uhlenbrock Daisy. This is a digital starter kit consisting of a handheld throttle and a control box, that can handle 16 locs and your turnouts and signals. When your're ready to expand to larger numbers or advanced functions, you can add an Intellibox and still use the Daisy as a handheld remote control.
Since the Daisy will hit the market this month, there is no experience yet, but I guess this will by my favorite digital kit.<img src=icon_smile_cool.gif border=0 align=middle>
If you are not in a hurry, you may want to wait for the reviews coming up at this site in the near future...