First I'd suggest you go to a good tackle shop like Michael and Young (Surrey & Vancouver), Sea Run on Austin in Coquitlam, Berrys in Richmond or Hatch Matcher in Maple Ridge and ask them to recommend some flies.
Second fish in the local Fraser Valley trout streams aren't usually too particular about the pattern or too selective. Get the size, shape and colour right and you should do fine. However there will be days that are exceptions. There isn't a lot of dry fly fishing but it does happen.
In general buy flies that generally represent what fish feed on:
Other fish: minnows - mostly stickle backs - smaller short shank bright flies - A tied down mylar minnow in #8-#12 is a good example as the Rolled Muddler
Sculpins (bullheads) - a wooly bugger in olive black or brown size #6 to #10 longer shank hook. Wooly buggers look a bit like everything (leeches big nymphs small fish crayfish etc) and exactly like nothing. The hackle and tail provides a lot movement so it looks alive. A muddler minnow and it's many variants is another good choice.
Salmon Fry - bright silver flies on longer shank hooks again a #8 or #6 hook. Salmon fry are about 35mm long and have thin bright bodies with dark backs. A tied down mylar minnow on a proper hook size is an old standby but most people use 'epoxy minnows' these days.
Nymphs: buy some generalized nymph patterns like hare's ear, Prince nymph, Pheasant tail nymph in various hook sizes - about a #6 up to a #12 or #14. Some of these should be weighted with a bead head.
Dry flies: something that looks like a mayfly - nothing better than an Adams for this and some down wing style flies like an Elk Hair caddis. Sizes #10 to #14 covers most everything. I also carry a few Griffiths Gnats in #14 or #16 to cope with the times trout feed on midges and black flies stuck in the surface film. A few ant patterns as well.
Midges/chirnonmids - basically the same as used for lakes in the interior but small.
Attractor Patterns: bright yellow, orange, green patterns - Mickey Finn, Professor, KCK etc #8 or #10. A few attractor Dy flies patterns like the Royal Wulff. To pick one size I'd go for a #10.
Get a couple of sizes in those categories and you are well equipped. I like to have at least 3 flies of a given pattern size and pattern as invariably when they start to work I'll quickly lose one in a tree behind me or in a fish. That way I have one to catch fish with and a back up.