Make sure your Sim's needs are in the green. Watch to see if Tony shows any signs of low needs (hunger bubble, sleep bubble, etc.). Continue with Romantic interactions without changing to anything else. Watch the status in the upper left corner. You'll see that Tony think's she's being friendly, being flirty, and finally being irresistible. As you progress, you'll start seeing new Romantic interactions unlock. Use them. Oh, also make sure Tony doesn't already have a girlfriend.