A transformer is an electrical device that transfers alternating current from an energised circuit to a circuit that otherwise has no current. A step-up transformer generates a higher voltage in the secondary circuit than in the source circuit. A step-down transformer does the opposite. In either case, the energised circuit magnetises material in the transformer, which in turn creates an electromotive force in the secondary circuit, thus creating another alternating current.
Strip the ends of two long (around two feet each) insulated wires. Scissors or a knife will suffice for this job.
Check the metal of the steel bolt or screwdriver you'll be using to make sure it's magnetisable. For example, see if a kitchen magnet sticks to it.
Wrap each wire dozens of times around the metal part of a steel screwdriver or a large steel bolt, leaving the ends exposed. They can overlap and the spacing doesn't matter. Keep track of which ends belong to which wire. Don't wrap them the same number of turns. Keep track of which has the larger number of turns. Denote the number of turns T and t, for the larger and smaller number of turns respectively. Your simple transformer is now complete.
The governing equation relating voltage and coil turns is V/T = v/t, where voltage V is the voltage drop across the coil with T turns and v is the voltage drop across the coil with t turns. Attaching the bare ends of the coil with the T turns to an AC source makes your apparatus a step-down transformer. Attaching the ends of the coil with t turns to the AC source makes it a step-up transformer.
Since your transformer is simple, not grounded or made for industrial use, use a low-voltage AC source, not a 110-volt wall socket. Lab supply stores sell AC sources that plug into the wall and step down the outlet's 110-volt output to safer levels. These AC sources are therefore also transformers themselves.