start by playing with the basic set, learn what works and what doesn't, it will take a few days at least. Learn from your defeats, use the tactics from the people who defeat you, and try and understand why, when you loose.
Card packs are sold out now, pre-sale about to start for the new chaos legion set, but it requires vouchers, that's too much for you for now, so start with buying the cheaper cards from the market.

you'd better buy from the monster market instead of the in game market, so you get a little Dec refunded.

While you're at it, read my beginners post: https://www.splintertalk.io/@nozem01/what-cards-for-beginners-and-some-tactics