Does it sound too formal to say "Please be aware that..." to a friend?

It's more that it's for formal circumstances (I often use quite "formal" English with one particular friend, but this is not the sort of thing we say).

I can't really offer a specific alternative without knowing what you're trying to tell your friend, but a couple of slightly more relaxed examples:

• Before we go, I need to [tell, warn, ...] you that I won't be able to stay for dinner.

• By the way, I just found out I'm allergic to shellfish, so please could we not go to Loch Fyne.

and last but not least [credit: @EdwinAshworth]:

• [name or Oi], you're standing on my toe!