Here's what I've found, and this is just my experience, so don't take this as gospel:
I can get very drunk while taking Lexapro (20mg) and have a blast. BUT, I would recommend, for now, not taking any alcohol. You're just starting Lexapro, so you want it to get in your system and help you, getting you back to normal conditions FIRST, as that's most important. After that and during a check-up with your doctor, ask him/her about alcohol and go from there.
In my case, I've been on it for years, and besides a goof a few weeks ago when I forgot it during a vacation and starting feeling symptoms, I've felt great -- my "normal self" -- for the same amount of time I've been taking the medication. I can say when I first started, I didn't drink, and ever since my goof a few weeks ago, I haven't taken a drink either. Gotta play it safe, and as I said, consult with your doctor once you're feeling better.