iPhone 3gs running 6.1.3 jailbroken - but need help - please

I have an iPhone 3gs that i recently upgraded to ios 6.1.3 (by a stupid mistake), after jailbreaking it, i am able to turn on my phone. but it doesn't recognize my sim card.
It seems to me that it should be possible to fix this, but i am unsure as to how. i have read a thousand tutorials, but i havent found one that seems right.
my baseband is 06.15.00

Also i am not able to install cydia - i know this should be done manually; but i havent found a working tutorial.

I am hoping that some clever minds can point me in the right direction, as i am not an apple/iphone expert myself. It is like navigating a jungle finding the right softwares, firmwares, tools, tweaks ect.
I am so confused and at a loss.

Please, help,
Amiara :)