Trichomes look like translucent sperm on the end of your plant. They are very sticky and get you very high. Some plants look like they've been snowed on they have so much (Look for picks of a type of weed called 'Ice').
As to when to pick, depends on the plant. If the ends of the Tri's start gettin brown, it's almost to late.
Look it up on the net, you'll find out.
PLUR