spring or fall?

You can only rush in the fall if you're a sophomore, so judging by your username, you're eligible :) as to who you know, that shouldn't really matter. the entire chapter has a say in picking new members, so while having friends in a certain group may help you, it might not.

from experience, i can say that during rush there have been girls that are friends with sisters that the rest of the sorority didn't like or didn't click with, so we didn't take them. who you know doesn't help unless you're a legacy (your mom, grandma, or sister and sometimes cousin was a sister of the same sorority).

rush whenever you feel comfortable. keep in mind that in the fall, only sophomores and up can rush, so you wouldn't necessarily be "competing" with all of the freshmen. also know that if you drop out of rush or don't get the house you want in the fall, you can always rush again in the spring. hope this helps! :)