How soon could someone get pregnant after an abortion? I'm scared to have sex because I've heard your most fertile after having a baby or an abortion.

Soon. Depending on how far along you were, it is possible to ovulate and get pregnant again within a few weeks after an abortion. You can start most almost any contraceptive method the same day as a first trimester abortion, including an IUD. Please see your clinician about starting an effective contraceptive method. Use condoms or abstain until then.
Any time you ovulate. I suggest you use a reliable form of contraception to decrease your chances of conceiving if that is your concern.