After playing the game for a while there are some things that I wished I knew when I started. This is not a all encompassing list, but just a few things I personally wish someone told me to make it easier when I first started out.
1) When you go to the "Cards" section it defaults to all the cards that are available to you. NOT the cards that you own. Which is a huge distinction. Cards that you own you can sell, trade for DEC, or combine.

The little red box is a drop down list, I would highly suggest you change it to "Owned" so you can see the cards you actually own.

2) Once you are sorted by your 'owned' cards you may notice on a card a little red banner in the top right corner of the card. If you card has a banner with a number that means you have that number of the same card. So for example the "Warrior of Peace" card in the top right corner, I have 2 of them.

Click on your card to see more details. Above the card details you will see 4 tabs "Cards" "For Sale" "State" and "Lore". I like to look at "Stats" to see what benefits I get for leveling up my cards. So I can see that at level 2 for this card it goes from 9 health to 10 health. And I will need 5 of the cards to level it up to lvl 2. I can also see at level 6 it learns a new ability called "Headwinds" which reduces the range attack of all enemy monsters.

You need to keep in mind though that even if you have a level 10 card your Summoner card dictates what level cards you can play. So even if you have a level 10 card it be brought down to a lower level during a match due to your Summoner card level.
Now I went back to the card overview and selected both cards by clicking their check boxes. Then I clicked the green icon that looks like its a deck of cards.

It will bring a pop up asking if you are sure and that once merged I will have 1 card that is level 2.
Here you can see that next to the card's name on the card it has a star and a number 2. That means it's a level 2 card. Right above that I can see the Health is now 10 hearts. And on the right hand side I see the value has gone up from $0.05 to $0.07 which is nice!

3) Don't forget to do the airdrop!
If you bought any packs of cards then you are eligible for the new card airdrop. You have a 1 in 65 chance of getting the new card after a certain number of packs are purchased. Go to the shop. It will tell if you if you are eligible for an airdrop or not. If you are click on the Claim Now button.

It will show you the new card and how many chances you have to get it. Click on the "Claim Airdrop" button and cross your fingers.

Since I have never got a airdrop card I can't show you what it looks like. But if you don't get one like me then this is the screen you will see.

4) To open a pack of cards took me so long to figure out. If you have any potions they will be used automatically and there is nothing you need to do to enable them. At the top go to the "Open" tab and you will see the screen below. If you bought a pack you will see the book that looks like it is bound by leather to be more colored in and have a x1 under it. All you have to do is drag it over to the blue circle next to it, in order to open it. Then wait for it to load and click on the 5 cards to turn them over to reveal what you got. Do not open packs unless you have potions, it is not worth it.

5) Go join a Guild. It's free to join and there are benefits to doing so. If you join a guild that has a decent level then you will get a bonus amount of DEC after each battle you won. By guild has a 2% bonus right now. It's not a lot but it's a free 2% increase.

This is my Guild and you can see the 2% DEC Bonus we all get

6) Befriend anyone in Splinterlands and maybe even in your guild. Because high level players get a crap ton of cards each season and I am sure will be more than happy to send you a few if you are nice. I haven't been so lucky and haven't received any but I have seen people here on Publish0x offer new players a few cards.