Where i can find more form?

[Yukito]

I have 1 form and fire form...

[FaeDyne]

I have 1 form and fire form...

You can find 3 more forms from bosses, usually at the end of large areas, and get 1 form without beating a boss. there are also a few useful costumes that you can only wear one of at a time that will help you in regular singing form.
It's not completely simple where to go, but by exploring you can usually figure out where you can or can not go.

spoiler: (tiny text. copy and paste to view, or view quote in reply)
south of the 4 "Open Water" zones is The Abyss, which is impossible to get through without something to light the darkness. there are 3 ways to do it, but only one of them is really good for it.
I don't know if I would recommend it, but I wouldn't say not to go to the southern 2 Open Water zones either. There is at least one useful item you could get now: the crab armor.

Always remember to keep track of areas you notice you can't get to at the moment, so you can recognize where to go when you get new abilities. example: now that you have beast form, you can swim against currents. any areas that you could only get to by current would be good to go to.

[Chibi]

What do you mean, "now"? You have to get the beast form to get the crab armor. Sheesh, faedyne.

[silverflagon]

It seems that it's not impossible to get through that area mentioned, it's risky and hair brained but at least one member has done it!

But there are 2 forms in the Kelp forest that will help you get further, and swimming to the east is a good idea also once you've learnt to use the pearls