How do u make a powerful electromagnet? What do u need?

An electromagnet is basically a coil of wire.

You can make it stronger by putting a soft iron core in your coil, increasing the number of loops, and increasing the current going around the coil.

B = \frac{\mu N I}{L}

Where B is the magnetic field, mu is the permeability of the core, N is the number of turns per unit length, I is the current, and L is the length of coil.

Most magnets saturate at 1-2 teslas, they are investing in 7T magnets for use in PET scanning, this is achieved with liquid helium (i think) cooling.