How do I get to Wesminster from Perth City?

Depends is you are driving or taking public transport...but ... in a car is easier to explain. You could head out of the city off the Freeway onto Charles St, which turns into Wanneroo Rd. Then it is just a matter of driving until you get to the area you want, which might mean a turn at Arkana Rd or somewhere up near Balcatta. If you want to take the freeway, then simply exit at Erindale Rd and then follow Balcatta Rd to Wanneroo Rd and then turn at the appropriate place to get to the street you need. The train and the a bus would probably get you to about the same place (or just a bus on Wanneroo Rd).