Here is a general deck building guide to help new players get started building decks in Magic Duels. This is in no way a comprehensive deck building guide. Building a deck can get pretty complex, but this guide should get you started.
You want about 40% land cards in your deck. So, for a 60
card deck you want 24 lands. Some decks can run more or less, but for now stick
to 24 lands.
The formuala is: total mana symbol count of a color / total mana symbol count of all color * number of lands you want in the deck.
So if you have 20 green, 30 red and 24 lands. To calculate how much green mana you need is 20/40*24 and red mana 30/40*24.
Aggro decks with primarily small creatures can get away with less land.
You may want to consider adding an exra land or two for decks with lots of big creatures or a control deck.
Creatures that tap for land can be counted as a half land.
I have some new player decks Magic Arena Deckbuilder here.
Here is a comprehensive deck building guide written by LegendVD check it out it goes into way more detail:
http://steamcommunity.com/sharedfiles/filedetails/?id=491003520
If you are a new player and havent unlocked many cards check out new player deck list. I have added a few starter decks and some Origins only decks. Also check out OriginMD's Deckbox, which has a number of starter decks with YouTube guides.
In addition, you can now search for decks containing specific sets on the decklist screen. You may want to check the archives for older decks from previous seasons.