One quick and easy way to build up circuits is the breadboard. They are available from many sources at various prices, size and features. You can add to this as you want. common add-ons are power sources, commonly used circuits, meters and what ever.

Power - You seldom need Amps of power. Wall warts are suitable. 5 V @ 300 mA to 500 mA, regulated for digital. +12 V or adjustable @ 100 mA or so for general use. Pos and neg 5 V to 15 V for op amps @ 100 mA.

For digital exercises you might find commonly used circuits useful. Clean switch inputs. Simple switch inputs. Pulse generators. LED outputs. BCD display.

Each of these will be covered in following posts.

 

Breadboards. I mounted mine on a wood box. Inside the box is power supplies (batteries) and add-on circuits built up on smaller breadboards. Toward the top of the picture is a variable (+ to - V) power source.

 

Breadboard power supply:

Make sure it will fit your breadboard. I have some medium size Radio Shack breadboards that this power supply does not fit. I have another large breadboard that I got at estate sale and the power supply fit very nicely.

 

Power
Batteries are okay.
Wall warts are a good start.
From wall warts you can build regulators to supply any voltage you want. Variable supplies are handy, preferably one that goes all the way down to 0 V.