I dont know much about tennis, but I want to get my girlfriend a nice tennis racquet thats light and high performance. What would you recommend?

Thanks!

I want to start playing tennis; mostly for fun and I have order a cheap tennis racket from Wal-Mart () . In the product’s details just said Wilson Federer Signature Oversize Tennis Racquet.
Is this a kid (junior) rocket?
How different are adult and junior tennis rockets?
Do I need to change it and get an adult racket?