After a while, you'll be asked if you're ready to order: Haben Sie gewählt? If so, then carry on:
When one orders food in German, the formal form Sie is used whenever possible to be extra polite. Don't forget your dankes and bittes either!
You don't have to use any complicated constructs if you don't want. Simply saying the name of the food followed by a bitte (please) will suffice. Otherwise, here are some other ways to order:
Ich möchte... I would like... This uses the conditional.
Haben Sie... Do you have... This just uses the polite “you" form, inverted to form a question.
Ich nehme... I'll take... This is more appropriate in a somewhat more casual context than a nice sit down restaurant, such as ordering from a street vendor, but it's not really considered impolite either.
Ich hätte gern... I would like to have... This also uses the conditional. Literally, it translates as “I would have gladly..."
All of these may be inverted by the waiter or waitress to ask you what you would like to order, for example Was möchten Sie? from Ich möchte, so be familiar with inversion and how to adjust conjugation according to changing the subject.
When you're done ordering, you'll probably be asked if you want anything else. Either continue on, or maybe just end it with a nein, danke.