hi i need to make a program in assembly mips that checks if a letter exist in one string any ideas?

Use of the SI register and the LODSB command would be a great place to start.
By letter, I assume you already know the letter that you are looking for?
I wrote a small sample for you in NASM, it uses the letter L as the letter to search for.

jmp main
string1		db "lookingL for a letter",0
main:
mov si,string1
findletter:
	lodsb	;Get current char in SI, place it in AX, then advance one position
	cmp al,'L'	;Is the letter L?
	je done		;If it was, we're done
	or al,al	;Check flags
	jz not_found	;If zero flag was set, the letter was not found
	jmp findletter	;Loop

done:
	;Jumps here if letter was found

not_found:
	;Jumps here if letter was not found..