'Judgement is a game mode that tests your might on the battlefield as well as your deck-building skills. You don’t use your collection of scrolls in Judgement matches. Instead, you create a bespoke deck [of 30 scrolls] from a random selection and use it for five competitive matches. Then you get rewarded depending on you performance.' Owen – @bopogamel
Judgement is Scrolls' version of a 'limited' format, which translates into the player having to pick from a limited range of scrolls that are presented to him or her in rows of 4. These scroll options aren't taken from a players own library, and a deck can have more than 3 of one scroll type. Whilst this is a great leveler between players with differing deck sizes, less experienced players may have problems with constructing viable decks from the scroll choices presented to them.
It may be worth waiting till you have a little more experience before you use up the free entry you gain in playing Judgement for the first time, however, a lot of players often prefer to repeatedly play this type of game than constructed. Their aim is to go 'infinite' which is to be able to win enough gold each time you play to pay for your next game of Judgement. Being awarded at least common scroll and 100g (the cost of a random scroll in the store) means that you wont go away empty handed but it's worth trying to get to at least 2 wins so you can bolster your library with a rare.
Combat, unit values, units attacking, and units being attacked is explained in this guide aimed at new players.
How to deal with the hard trials in Scrolls when you have a small library.