Step one · Hebrew calendar
Nobody tells you this part clearly. Put in your date, get your Torah portion, your haftarah, and the Hebrew date you'll see on the invitation.
The Torah is read on a fixed annual cycle, one portion per week. Your date lands on a specific Shabbat, and that Shabbat already has a portion assigned. You don't pick it. It picks you, which is honestly more interesting. Calendar data comes from Hebcal.