One of the big mistakes I've seen from new players is sacrificing for scrolls when they should be sacrificing for resources. I'm going to explain the different situations you will encounter during play, and how I generally deal with them.
You should be sacrificing for either scrolls or resources every single turn, unless you are confronted with a situation where you are forced to play your hand without doing either. For example, you may have a Burn and Spark - which you desperately need to get rid of a Great Wolf.
At the start of the game, you should always be sacrificing for resources over scrolls. If you don't build up your resources early game and your opponent does, then you will not be able to keep up with their field presence. This will allow them to have control over the game and you make things unnecessarily harder for yourself. As a general guide, I would say that sacrificing for scrolls when you have 5 or less resources is incredibly dangerous. That's not to say that you should stick to 5/6 resources. I usually try to get to 7 then focus on sacrificing for scrolls unless I have a decent opportunity to sacrifice for resources.
While it can be daunting to continually sacrifice for resources while your enemy is playing more creatures at the start, eventually you will get to the point where you can play 2 or 3 scrolls a turn (depending on their cost). In these early stages it is vitally important to make use of any scrolls that will let you draw scrolls, Sister of the Fox, Burn and Pother are a few examples of low cost scrolls which allow you to draw. This will give you more options when it comes to sacrificing.
Sometimes it is necessary to gain more resources even if it means you will have no scrolls in hand. However, this will also largely depend on your opponent. If they have less resources than you, then it is safer to sacrifice for scrolls (although not ideal) if you really need to. It can also be tempting to try and draw into a certain scroll. For example, if your opponent has played a Kinfolk Brave – you may want to find a Spark to get rid of it. But it's much more worthwhile to sacrifice for resources, play a structure, then a large creature and deal with the threat when you can.
During the late game you should be keeping a close eye on your opponents resources. Make sure you don't fall behind them by too much. If they are on 8 and you are on 7 it should be ok, but if they're on 10 and you're still on 7, then that's potentially 1 more scroll a turn they can play (an extra Burn, Skirmisher, or Mangy Wolf coming your way!).
If you keep this advice in mind when playing you will be in a strong position and will be able to make the most of the scrolls in your hand.
Combat, unit values, units attacking, and units being attacked is explained in this guide aimed at new players.
Nrp's guide on how to beat the hardest of hard trials