Yevgeni,
If a capacitor is marked like this 123, it just means 12+3zeros = 12 + 000 = 12000pF = 12 nF. And that's exactly the way you write it too. Value is in pF (PicoFarads). The letters added to the value is the tolerance.
D -> 0.5pF
F -> 1%
G -> 2%
H -> 3%
J -> 5%
K ->...