im looking for the first kernel of linux but it has to be a iso file so i can use it in vurtual box

As the first kernel was written by Linus Torvalds there were no existing distributions and the use of CD-ROMs was not common. So it is very unlikely that you will find any ISo that comes with the first version of the Linux kernel and is bootable from an ISO.
Having said that, you can get the first version (and any other version) here: http://www.kernel.org/pub/linux/kernel/Historic/

well how could i use it in vb then

You would have to compile it (with an old GCC) together with the needed software, not unlike Linux From Scratch.

You might want to read some historical articles first:

• A Short History of Linux Distributions
• History of Early Linux Distros

That being said, oldinux.org has some pretty ancient stuff, including some hdd images for emulators (mostly Bochs). But that most likely won't work with VirtualBox.

As Tobi said, the earliest kernel versions weren't really distributed on CD media and at first there were no distributions. But, what most call the "first distribution" were boot/root disk images distributed by HJ Lu. You can also try out those from oldlinux.org archive. I don't know for VirtualBox, but I think they worked on QEmu last time I tried.