There's an option to "create a sub" on the sidebar of many subreddits (some subs may choose to hide the option). Then the sub must be promoted for people to know about it, and it has to appeal to enough people to grow. 

Silver, gold, and platinum are "rewards" users receive from other users for writing good posts/comments. Gold and platinum have a few perks. They cost money to buy so they help keep the site running.