Here is a tutorial designed to give beginning DAW (digital audio workstation) users some ideas about what plug-ins are out there and some ways you can go about choosing the right plug-ins for you...
As always when choosing new gear to add to your rig, the most important aspect is knowing exactly what sort of use you want them for! For example, what type of plug-in effects will be useful for you? For what sort of purpose? What is your budget? Should you get a plug-in bundle or buy them individually? These are the sorts of questions you should be asking yourself. Of course there are a wide range of users out there; some are looking to add a number of plug-ins to their system to get started with, and others might just be looking for a plug-in or two to add to the collection. No matter what category you fall under, always consider why and for what purpose you're getting these plug-ins for.
Another major consideration when deciding what type and brand of plug-ins to buy is what type of system you currently have. Some plug-ins will only work with certain pieces of software, while some require some sort of hardware to go along with them. Plug-ins come in a variety of different formats, and you need to know which format(s) your current software will support. For example, Pro Tools will only run RTAS and AU (audio unit) plug-ins and will not support VST. Some will only support VST, and so on and so on. For Pro Tools users, FXpansion makes an VST to RTAS converter, which I believe is a great investment. Another thing to keep in mind is that HD and LE type systems will require different plug-ins, so make sure that if you're running an HD system you get the TDM plug-ins and if you are running LE you get native plug-ins, as there is a huge difference in price and they aren't inter compatible. Identifying where you fit here is very important, as you don't want to end up with a plug-in you can't even run!
A lot of sequencers will come with plug-ins of their own. For example, Pro Tools comes with a number of plug-ins already installed to get your feet wet with. While they aren't the best plug-ins out there, they will give you an idea of the different types of plug-ins that might be available. I know that pretty much any sequencer will at least have a few plug-ins available, some of course being better than others. Knowing which of these free plug-ins will help you and which won't is important, as this can save you a bit of money if you don't have to buy a plug-in you already have a substitute for. From here you can decide what types of plug-ins you might want to get...
While this is quite brief, I'll mention a few different types of plug-ins in terms of different signal processors. You've got your dynamic processors, which includes compressors, limiters, gates, de-essers, and so on. Having at least one compression plug-in is really a must for mixing purposes. Pretty much all sequencers will have some sort of built in EQ, but there are tons and tons of equalizer plug-ins out there which can go well beyond this. I prefer using parametric EQ, but you've also got graphic EQs to choose from. Then there are always your reverb and delay plug-ins. Having a reverb plug-in is an absolute necessary in my opinion if you are going to be doing any mixing. They help to space your sound out and simply add a ton of options to what you can do inside your DAW. In addition to these types of plug-ins, you've also got modulation effect plug-ins like chorus, flanger, and phasing. These are probably the least important for mixing, but can certainly be nice to have around. The list of different types of plug-ins could go on and on, so I'd encourage you to browse the different plug-ins out there.
Lastly, I'd just like to say a note about free and trail versions of plug-ins, as these can be incredibly helpful! There are tons and tons of free plug-ins out there, most of them in VST format. Most plug-in manufacturers will offer up some sort of a free trial version, which I highly recommend taking advantage of as it will give you the opportunity to shop around and see what will work best for you...
Hopefully this guide will give you some help about the different options available to you when it comes to plug-ins!