so what should i do in order that i can use my transistor as a switch?
Current can't flow unless there is a Voltage and there is No Voltage without a Current.Even when measuring with a multimeter, there is always a very small amount of current passing from the device under test and the multimeter.
Every circuit requires a certain amount of voltage and be able to draw a certain amount of current to operate properly. Although there may be a voltage across the Base Emitter junction, there my not be enough current supplied from the Base Emitter junction for the circuit you wish to switch on. By altering the current that can pass the Base Emitter junction changes the amount of current from Collector to Emitter based on the Hfe of the transistor.
For example If you want to switch a LED On or Off with a transistor at 12V. The LED will light brightly with 12V at 15mA. The transistors Hfe is 100.The transistors base is restricted to only 1 uA at 12V. so 1uA times the Hfe of 100 is only 100uA. Not enough current to light a LED brightly. If you increase the amount of current to cross the Base Emitter junction to 150uA then 15mA will pass across the Collector to Emitter. 150uA times the 100 Hfe of the Transistor gives 15mA that lights the LED brightly at 12V.
Another way is by restricting the current base of the transistor to 150uA with a base resistor. With no voltage applied to the base no current will flow. When 12v is applied to the Base 150uA flows across the resistor to the Base Emitter junction. This turns on the transistor allowing current to pass from Collector to Emitter. This turns on the LED brightly at 15mA at 12V.
Theirs literally Thousands of videos and web sites explaining how to use a transistor as a switch. Along with the math and a knowledge of Ohms Law you can predict what your transistor will do.
Just remember to keep the amount of current to the base of the transistor low.