It's been a while, but there should be a tutorial at the beginning. Next, try beating the AI in bronze, silver, and finally gold mode. For ranked play, you can find a lot of ideas for building decks, such as https://www.rschronicle.com/forum/posts/list/6808.page

What really helped me when I started was watching some youtube video's on beginner decks.
I watched the deck guides by The Killer Bits and Merchant (he is a dev of this game).
They explain the thought behind the chosen cards really well.
Maybe if you search around there are more good channels but those are the ones I watched and learned from.

Edit: And look for the more recent vids, the older ones may contain reballanced or reworked cards.