dB is logrithmic so it is alot easier to work with these as you only add and subtract dBs.
Antenna gain will only ever be given in dB (as far as I know) and only applies specifically to the antenna.
EIRP relates to the total power emitted from the antenna which is
Radio Tx power(dB) - cable loss (dB) + antenna gain (dB) = EIRP.
The only bit you "should" need to convert will be the radio Tx power because that may be in mW.
10dB = 10mW
3dB is half or double power depending if its +/-.
so 13dB is 20mW 16dB is 40mW etc.
20dB is 100mW.
You will be fine tomorrow mate
No, e.i.r.p. is not the same as antenna Gain. The term equivalent isotropic radiated power or e.i.r.p, is a function of the antenna transmit gain G(T) in dBi and the transmitted power P(T) in watts fed to the antenna. It is written as e.i.r.p. when used in text and as EIRP when used in formulas.
EIRP = G(T) + 10 Log P(T) (in dBW)
The value of P(T) is the sum of the transmitter output power and all the loses/gain in the path from the transmitter to the antenna.
The gain of antenna is typically in dBi, while e.i.r.p is in dBW or watts. In wireless WAP i believe they reference mW or Watts.