apropiate iso and architecture, whats that?
 

I decided to go linux and chose gentoo and blablabla. When downloading which architecture and iso should I choose and what is it?

i don't get that part :\

anyways , for most computers i386 or i686 is fine if you have 32 bit processor. if you have an 64 bit processor and at least 2GB ram then go for the amd64 version.

though since i see you're a begginer and don't even know the difference between architectures i recommend you a simpler distro , like ubuntu , or linux mint.

Concur; gentoo is a 'build from src' distro, generally aimed at advanced users, although it's your choice to do whatever you want.
(For 64 bit I'd want at least 4GB RAM to make it worth while)
Have you had a look at distrowatch.com?