I'm late on responding, but yay brook trout are my fave with fly fishing!
(see <<picture for my mini brookie)
i'm not good with fly names, but in late april when the season opens i usually use these small white/light grey (kind of fluffy) flies, or adam's fly is classic as well as one that looks like an adam fly but has a red shiny band around it.. you can usually see the bugs they're eating off the surface and try to match them if you can.
*however this is for a small river, i've never fished for them in a lake before...
Good luck and make sure you post about it afterwards!