New to the game, and I don't really understand how versus battles work

Idk if this is a silly question or not, but when will I get the weakness advantage? I have tried 8 versus battles with different decks and my opponent has had weakness advantage every time. I can't find any info in the rules on this.

Also, why am I only going against people with 40+ levels on me? Their ex cards ruin my stage 1 pokemon instantly and this game is losing it's fun fast.