please forgive me for being so blunt about it but I feel I have no other choice, as I feel obliged (or at least try) to prevent you from harm.
You are building an electronic device which you will plug into your main power supply, not a toy or a little 9-V-effects pedal. You need a little bit of understanding of what you are doing and especially a little bit of experience. Although the Madamp-documentation is very detailed and almost fool-proof, meaning very friendly for less experienced customers I (in my very own personal opinion) find it grossly negligent to build a tube amp without any experience at all. Being able to identify the parts is an absolute basic.
Regarding your question: "Where is positve?" Well, on the opposite side of the gigantic "-" which indicates negative
I am pretty sure you will not want to hear it, but I say it nonetheless:
My suggestion would be to stop building the amp right in this moment and to order some low-volt pedal kits first. If you know what's what, just continue with your build. An alternative would be to read a lot (books, internet, forums) and then resume your build.
Don't get me wrong, I am not saying that you should not build an amp. I am just saying that at this moment it is very risky as you don't have the basics (yet).
Cheers and kindest regards,