All you have to do is divide both numbers (ones they both can go in).
Let's try 2.
[tex]6/2=3\\18/2=9[/tex]
We get [tex] \frac{3}{9} [/tex].
A number still can go into both numbers evenly.
3 can go into both numbers.
[tex]3/3=1\\9/3=3[/tex]
We finally get [tex] \frac{1}{3} [/tex].
Hope this helped :)