Is there a brick that I can use to connect lego plates at a 90 degree angle?
There are many. I believe the classic "Erling"/headlight brick (it was designed by a guy named Erling) is the oldest way, but there's also a number of bricks with studs on the side and a number of brackets.
The part# 87087 - "Brick, Modified 1 x 1 with Stud on 1 Side" could work:
But the part# 11211 - "Brick, Modified 1 x 2 with Studs on 1 Side" seems better to scale up with: