Here's a pretty good guide for the card system on AST. https://www.reddit.com/r/ffxiv/comme...ayers_content/
As the others mentioned though, you should AT LEAST level your Conjurer to level 8 and get Protect. If you don't, expect to get a lot of hate your way. It doesn't take long to level a CNJ to lvl 8 (like maybe 30min-1hr tops) and Protect is one of the most basic and necessary things a healer should be doing as soon as they begin any dungeon/raid/etc.