help please

i am new here and i am a new user of unix.my problem is :
i want to find all the names of files under the directory /etc/and at any depth that contains the letters 'st'.how i can do it with using
a)ls
b)find

for (a) i am thinking something like ls -R|grep *st* but is wrong

any ideas ?????????

with find :

find /etc/ -type f -print -exec ls -lR *conf* {} \;

• replace conf with your string.

find /etc/ -type f | grep st

thanks dudes