I want to charge Lead acid battery (12V, 55Ah) I have adjustable DC power supply (I can set voltage and limit amps). Do I need to put resistor in between power supply and battery? If so.. how much Ω ?
Resistor will not be required, you may limit the current to 1/10th of the battery Ah value, or at the most at 2/10th of the battery Ah value, above this it is strictly not recommended for a lead acid battery. Next, let it charge for 10 to 12 hours minimum, and then remove it from the supply, you will find it optimally charged.
The battery spec sheet (and often, a label on the battery itself) will specify charge voltage and max current for float and cycle use.
Float use uses a lower voltage and current may be unlimited (as the battery will set its own limit at Vfloat.
For cyclic use the voltage is higher and Imax will need to be set.
See (and bookmark) Battery University for a vast amount of information on batteries - lead acid and other.