Don't tell her "I love you". Not yet, not before a few more weeks at the very least.

As strange as it may sound, it's one of the best way to scare a girl away. If you want to make things official, tell her so. Tell her you'd like to get closer, etc, etc. Make the intent clear that you want her to be your girlfriend. Give it some feeling too, but no mushy corny stuff. She doesn't sound like the kind of girl into that.

Now if she says it that's another thing.