If you already have a decent assortment of electronic components on hand like resistors, capacitors, transistors, LEDs, switches, potentiometers, etc, just buy an Arduino Uno.
If you don't, you might get a kit, such as
this one by Vilros.
Elegoo offers similar kits, and I imagine other suppliers have them, too. The Vilros offering comes with a very good instruction book filled with exercises and general electronic info.