I always have been taught to brush my teeth first and then after brushing my teeth I then floss my teeth with dental floss to remove any excess food particles from in between my teeth.
You can floss your teeth first if you want too and then brush your teeth because it doesn't really matter but what works best for me is to get my teeth brushed first.
Then after brushing my teeth and getting them really clean I use the floss to remove other particles that are left behind.
I also floss my teeth after every meal as well to keep my teeth clean and get food out of my teeth as I can't stand to have particles of food sitting between my teeth.
I use mouthwash at least once per day as well to keep bacteria out of my mouth.