To create your recurring event's regularity touch on Repeat and choose Custom. This is in the third box down, the first two being the title and the all day or start/end time and travel time.
After touching on Custom choose Frequency Monthly.
You will see in the lower box the label "Each", with a tick because you chose it in the previous screen. Below this is the label: On the...
A list of dates is below this.
Touch on the words "On the..." and this will reveal a new set of options for the first to fifth and the last occurrence of the day you can select on the pick wheel to the right of this.
Then at the top left of the screen tap on < New Event to go back a screen. Here you will see the selection you just made. Then at the top of the screen tap on Add to create your event.
But - if you want a recurring reminder you can use exactly the same set of steps to crate a recurring reminder in the Reminders app on the first Friday of the month, to use your example.