Before you jump into the deep end of Hanoi street food (which you absolutely should), take a survey of the offerings in a low-stakes setting.
Quan An Ngon is very well known on the tourist trail, but it serves an important function. Food stalls surround the dining area in the open-air restaurant draped with tents and twinkling lights. Before ordering from the long menu of classic Vietnamese dishes, walk past the cooks and piles of ingredients to see the dishes coming together. Ask the servers to help narrow down the options, then order a table full of plates to share.
Now that you have your bearings, go out and find some real street food at 1/20th of the cost.