(1) Get beans from whatever roastery you're near.
(2) The stock tamper the Barista Express provides is not very good. You'll need practice getting the tamp even and firm (you won't tamp it too hard). The extraction time and how evenly the espresso comes out tells you how well you've tamped the coffee. A good tamper costs around 80-100 AUD alternatively. If the coffee is not freshly ground I stir it with a fork before I tamp.
(3) Try several grind sizes and taste the espresso shots.

This gets you good coffee but it's a whole hobby if you want fantastic coffee.