Virtual instruments are one of the elements that, in my opinion, have revolutionized home recording. It's enabled dreamers like me that dreamed for 25 years of recording songs with instruments like sitars, saxophones and steel pans and many others to do so. But the existence alone of the VSTis doesn't guarantee glory. Because the real heavy weather is twofold ~ hunting down the VSTis and testing them out and then learning how to tweak them to realistic ends.
I've got friends that play steel pans and I was about to buy one a couple of weeks back. I'm glad I delayed while looking for a 14 note pan because I found this fantastic sampled steel pan for free ten days ago. It is fantastic, it is not some synthesized fake, but a real played pan. The reason I mention this is twofold ~ firstly, there are some good free ones out there in cyberspace. You have to look and find and be disappointed. The same applies to the ones you buy. People will recommend but all our tastes are so different. I have an almost monomania about instruments sounding authentic. Sampling was great for that because at least you were getting the genuine instrument. But in order to get to the point where you are pretty much settled in what you have, you have to go through alot of packages and searches. It takes alot of time.
I have the software sampler Sampletank 2. It comes with more instrument sounds than even Quincy Jones and Queen could use in a lifetime. These sounds are actual samples of actual instruments, rather than synthesized versions of instrument sounds that didn't really sound accurate. When you play the sounds on Sampletank you're getting those sampled sounds. Now, to me, they are VSTis, which I understand to be virtual instruments. I have other VSTis such as Celtic Instruments, Danny Thompson double bass and Zero G harmonica which I have loaded into Sampletank. They contain single notes so you can play whatever melodies or notes you want to play. They also contain riffs and melodies that I suppose you can work into whatever song you're recording, if you are that way inclined (I tend not to be). On the other hand I also have Cubase 5 loaded into my recording computer (I used to have Cubase SE) but because I use a standalone DAW, I use Cubase to house my other VSTis. So loaded into Cubase is First call horns, MTRON (a mellotron), Lounge Lizard (electric piano), B4 (Hammond organ), Miroslav Philharmonik (orchestral instruments) and Swar systems (Indian instruments). You could add to that Sonik synth and Garritan personal orchestra. These give me a range of colours to add to my recordings. They vary in quality, but again, to me they are VSTis (virtual instruments), though they are referred to as samples and sometimes sample sound libraries.
Basically, if you see the terms 'vsti', 'samples', 'sample sound library', 'virtual instrument', 'sample based' ~ investigate ! I made the mistake of buying this, which I thought would be a great addition. They said it had samples of Indian instruments etc but when I got it, it wasn't quite what I thought. It was mainly loops and melodies and though I was able to load the instruments I wanted into Sampletank 2, the range was only about 15 notes. Usable, but harder work, than say, Swar systems which, for most of the melody instruments gives at least 3 octaves.
But some DAWs come with VSTis. Cubase 5 came with hundreds of sounds from Halion. But I personally thought they were all fairly lame except for some of the synthesizer and sound effects sounds. So I don't use them. It's hard to say "this sound is better than that sound" because it's really so much down to personal taste. I know what I like. For instance, I used to use the violin and viola sounds in Sampletank 2 until I got Celtic Instruments and Miroslav philharmonik. They're so much better that I'd never use the Sampletank ones on their own again. There's so much variation.
Bear in mind that there are also loads of free samples and VSTis on the net and there are lots of people on this forum with invaluable information on them and where to get them and how to load them.
When you put your sample CD/VSTi into the computer, a menu should appear or something that asks you if you take on board all their terms and conditions. As long as you say yes or "I agree" then it should take you to the installer. Basically, most of the ones I have moreorless load them for you. The ones that go into Sampletank 2 are a little more complicated but believe me, I'm technically challenged to the max and I've managed to figure it out.........eventually ! Sometimes, it's taken me months.
Anyway, once loaded, they should show up in [your DAW]. If you press F11 on your computer keyboard, the list of instruments that have been loaded should show up. As long as your MIDI/controller keyboard is properly connected, just click on the instrument you want to play and the menu of those instruments should appear. As an example, say I click F11 and a whole list of programmes (MTRON, Lounge Lizard, Miroslav etc) comes up and I want to play some mellotron. I click on MTRON. Then all the different mellotron sounds come up. Say I want the half speed Hammond mellotron, then I click on that and Bingo ! Whatever you're monitoring through (headphones, speakers, whatever), you should now hear loud and clear whatever you play.
And importantly, read the manuals. They make bits of sense that help you to join the dots !
I hope that's a useful start.
I would try to combine real instruments with virtual ones if you can. It's the combination, I find, that prevents the virtual ones standing too much scrutiny !
Hope this all helps.