Kimkash, I cannot stress enough how important it will be to you to read up on those links that LarryD gave you. There are many kinds of transistors. People generally, but not always, mean Bipolar Transistor when they say Transistor. But it is bad form not to specify, it makes it more difficult for people to help you.
For instance, a MOSFET does not require a resistor between the Arduino and its Gate. However, MOSFETs have a large Gate capacitance so if you are trying to use the PWM Analog output of an Arduino to drive it, you may find it gets hotter than you'd thought as it takes time to turn on and off.
A bipolar transistor does require a current limiting resistor from the Arduino pin to the Base lead. You do need to take Beta (current gain) into account, especially when switching larger currents. Bipolar transistor gain goes down when saturated, so with a rated Beta of 100 you might find that you need 1/20th to 1/10th the current going into the Base.