How to control 220v ac lamps using 5v dc safely
simulate this circuit &ndash; Schematic created using CircuitLab

This shows Rb (base)=R1 which due to low current gain of 10 to 20 at low Vce(Sat) for transistor.

Take coil resistance spec or V+/Icoil=Rs and use R1 = 10~20 * Rcoil.

Some 5V relays draw more current than CMOS on Arduino can drive.  Relays must be rated for Lightbulbs like motors take ~8x surge current when switched on.