Romaine Lettuce

Okay, so for the first time this year I am growing Romaine Lettuce in my garden, only problem is, I'm not really sure how to harvest it.

Do you pull off a few leaves each time you want a salad? Do you take the whole plant? Will it continue to produce all summer?

I'm thinking that I should start picking tonight. I'm a little worried about my one plant - it looks like it has a new bunch growing out the top and has a "flower" emerging from the middle as you can sort of see in this picture (just realized with the onions in front and tomatoes behind, this picture looks a little overgrown and wild). Is that normal?