Lexapro is an SSRI.
SSRIs tend to take a while to have an effect - even the ones with a very short half life.
It's been less than a week, so you won't notice any serious changes. You probably won't notice any serious changes when it's already working, other than that you may no longer be shutting yourself down or whatever your depression causes you to do.
I took Lexapro once when I was a child around... 11 years old? 10? Dunno anymore, it's been a while. But it worked, though the medication also had the bad side effect of absolutely ruining my mental capabilities. I also tried Zoloft, which changed my personality and gave me severe heartburn. Eventually settled on Prozac, which worked just fine.
Everyone's different - SSRIs are safe enough, and there are several of them. Wait until you've at least been on it for a month under the supervision of a friend or a family member, see if they notice anything different other than the depression going away. If there's a visible side effect, tell your doctor, get on a different medication.