Need help about SEH.

Hi, I'm learning SEH things and I'm bit stuck on it.. I have some questions and can anybody help me with it ?
1) How SEH works ?
2) How can I initialize SEH chain ?
I have written little proggy here but I don't understand alot from there Smile

Code:

proc main
     push msgbox
     push dword [fs:0]
     mov [fs:0],esp
     mov eax,0
     mov eax,[eax]
     ret
endp

proc msgbox
invoke MessageBox,0,msg,ttl,MB_OK+MB_ICONINFORMATION
jmp fixa
ret
endp

proc fixa
invoke MessageBox,0,msg1,ttl,MB_OK+MB_ICONINFORMATION
ret
endp

I found some example on web and trying to understand things..
I'm pushing procedure which will occured when exception will happen, after that pushing [fs:0] which is SEH I guess and then moving ESP (SEH address) to [fs:0] but I don't understand why... Also, I'm trying to not destroy stack but after adding these instructions forward from mov [fs:0],esp and then add esp,8 - exception says can't read address from 00000..
Can someone explain me how to do it without destroying stack and how it works ? Thank you.. Smile