First up, in Lightroom there are no “actions”. They are actually called presets, although they do the same job.
Here are 15 things you ought to know:
1. To create a new preset, simply process the picture the way you want it and then click on the “+” in the left menu, next to “presets” and simply choose the settings you wish to add to the preset and it's name. Once you have saved it, you can find it again in the folder you saved it in.
2. A preset can be a very powerful tool when you have to process batches, for example if you are shooting with a mobile studio and don't need or don't want to process every image independently.
3. You can also use presets for a special effect you use regularly, for example a certain black-and-white effect, highlighting a specific color, setting a certain value for saturation, vibrance etc.
4. There are quite a few nice presets available by default. In develop mode, in the left menu there is a drop-down called “presets”. Give them a try!
5. You can hover over the name and instantly see the effect in the navigator. Sure, it's not the biggest preview but you can at least decide whether it could match your purpose or if it's just not right. As soon as you click on it the changes will be applied to the picture and you can decide properly.
6. As an alternative you can use the “before/after” view to apply presets to the whole picture and see what it looked like before in comparison.
7. Once you have used a preset you can always tweak the given values. You are not stuck with them.
8. You can download and install presets from the internet.
9. As I said earlier you can save everything in one preset. Sure, this works when you're doing a certain project or a certain batch. But when it comes to effects you shouldn't add too many settings into one preset. You could for example make a preset for adding a grain and adding a vignette. But if you want to only add a vignette to a picture, you're going to have some work to do to get rid of the grain again. A preset is made to reduce the work you have to do, so instead of putting different effects into one preset, split them and make several.
10. You can apply presets during import! Therefore it is a good idea to have one preset with the settings you change on almost every picture. I tend to increase the clarity for example, so I have one preset that I always apply on import meaning I have one step less in my processing.
11. Another way to use presets is for inspiration. If you have a good picture and can't quite figure out what's missing, try different presets. You can always tweak them or undo them if you don't like it.
14. You can store your presets with the catalog. To do so, go to Edit → Preferences → Presets and tick the box in the middle.
15. You can very simply update your presets. Just right click on the preset and choose “Update with Current Settings”.