Dakuten is actually really easy to learn once you've learnt the basic hiragana, you just have to remember which groups change really, the sounds that they change too are also generally similar to the original sound for example sa, si, su, se and so change to za, zi, zu, se and zo, very similar if you ask me, you just have to remember if its the speech mark like things that changes the sound or the dot (or both). It's pretty simple really!