"Silakan" would be something that you're going to say as the Waiter, not customer.
If you want to be polite as a customer, you can just say "Aku mau/ingin [makanan]"
Want to be more polite? Do it in a form of question like "Bolehkah saya memesan [makanan]?"
"I would like to..." is just roughly the same in Indonesian with "I want to...", yes this is also polite.