Factors that can make a electromagnet stronger are increasing a coil's current flow, adding an iron core and increasing the number of turns to a coil. Building an electromagnet involves wrapping a copper wire around an iron core like a nail. A battery is then attached to this setup so that a current can flow through it.
This process is a common demonstration of how an electromagnet works, which is used in physics labs. If students increase the number of turns of the wire, it will increase the electromagnet's strength. Similarly, increasing the current through the electromagnet will accomplish the same thing. However, there is a limit to the increase in current because this can cause the wire to become extremely hot. For this reason, it may be a better option to increase the turns.