Many new players do not know to upgrade their collection or how they can get "better" cards for their deck.
There are 3 solutions for this problem if you do not want to spend money:
Level Up: When you play a game an amount of XP is stored to your level. When you finish a level you get a pack. To upgrade your collection you can try to play more matches -> get more XP for your level. At every level, you get 1 core rare pack, at every 10 levels the core rare pack is changed to a core epic pack and at every 25 levels, you get a legendary pack. If you do not have luck, you will get for every 25 levels at least 1 legendary, 2 epics, 22 rare and 100 common cards, but base on my experience I tend to get 1 legendary, 4 epics, 30 rare and 90 common cards for every 25 levels.
Weekend Rank Rewards: Each weekend there is a rank event that rewards players for their first 25 matches. Based on their performance, the players can get free packs. A table of the rewards can be seen here.
Besides the first 25 matches, players can win 30 games to get another rare pack from the current set.
Star Store: Every player has access to the Star Store. The Stare Store is a marketplace that sells core and welcome shiny cards. In the store, there are 1 common and 1 rare card from the core set and 1 meteorite and 1 shadow card from the welcome set. For the new players, I recommend buying only the cards from the core set (mostly because you can get cards that you do not have or you can sell them after you forge to better quality). As a currency, you can use only stars to buy cards. Every day the common card is changed and every week the rare card is changed. As from my experience, I buy only cards that I see in my matches.
Examples of cards that i bougth from the Star Store:
 |  |  |
|---|---|---|
 |  |  |
 |  |  |
 |  |  |
 |  |  |
 |  |  |
If you want I can share my starter decks when I started playing Gods Unchained and how I upgraded them over time. If you want this, tell me in a comment. (I played War and Nature)