Reaper's "ReaTune" is an absolutely marvelous plugin given that it's part of just the always excellent Reaper package. If you use Cakewalk, for example, the DAW might be free, but it tries to sell you Melodyne after a (fairly useless) trial period -- Melodyne's cheapest option ("Essential") on its own, is US$99, and does largely the same things as Reaper's ReaTune. Reaper in total is $60. $60 for vocal tuning plus all the other Reaper magic. D'uh!
Ok, Reaper's ReaTune UI is not quite as swish. But trust me, you can use it and it works amazingly. Kenny's video (mentioned previously) is great. Just remember to bring down the Attack to about 100 ms as he mentions, maybe play around with it to get the best result for what you're trying to do.
Why am I posting really? Surely not to repeat what others have already said? Well, no, I was going to talk about key identification. Kenny assumes you know the key in his video. But you can do one of three things if you don't know.
1) Set the scale to (anything) Chromatic -- for non music theorists, that's every note -- all the white ones and all the black ones. That will work to give a basic polish any mostly decent vocal.
2) Get a plugin that can do key detection (Mixed In Key have some software that does this -- it's aimed at DJs really, but it kind of works, mostly...). I'm not aware of anythng that can do it that's already implemented in Reaper.
3) Without meaning to be unkind, you might find it helpful to bite the bullet and learn a little bit of music theory. I would suggest that it's possibly easier than you think. Let me suggest what to do -- and I'll be nice and friendly (I've seen some music snobs rant about this on other forum posts -- I won't do that!)
So, it's often cheaper and quicker to just figure the key out for yourself. I have the advantage of being a classical pianist, but I think my advice will work for most producers: Try this :
a) Assuming you can identify the note "C" on a piano keyboard (if not, look here:
https://www.youtube.com/watch?v=HaA3_PXx6Mo), play all the white notes in an upward direction, so that it plays the irritatingly useful "doh-re-mi-fah-so-lah-ti-doh" tune so popular amongst Sound of Music fans. Play it a few times, learn how that tune sounds. "La" it to yourself in your head. Imagine that it's your fave bit of music ever, so you just know it by heart. "la-la-la-la-la-la-la-la"... "doh-re-mi-fah-so-lah-ti-doh". Just learn this, Ok?! Brilliant. That tune, when it starts on C, is in the key of C Major. Got it? Great -- we can move to step 2. Don't forget how that tune sounds.
b) Once you've done that, try playing that tune so that it sounds like the same melody, but starts on a different note. Perhaps slightly higher, or slightly lower. The note the tune starts on is the Major key in every case. Let's do an easy-ish one as an example -- try it starting on "G". You will find that to play the "doh-re-mi..." song you have to hit F# (F sharp) instead of F.
c) Once you've practiced doing this, then you can try playing it alongside any bit of music you're interested in. Now, it'll be tough if you're going for hardcore jazz or some complicated classical, but if you're going rock, pop, or dance or something, you should find that you can find a version of the "doh-re-mi..." song that sounds in tune with the song you want to learn the key of. Try different ones until you find one that seems to have all the same notes as they're singing (or trying to sing!). 99 times out of a 100 you'll have nailed it. Look at the note that "Doh..." lands on -- that's the Major key of your song. Easy right?
Now, this does get trickier with Minor keys. Each Major key has a relative Minor. In the case of C Major, for example, its Relative Minor is A Minor. You can work out any Relative Minor by stepping backwards down the keyboard three semitones (ie the next adjacent notes, including the sharps and flats, three times). The Relative Minor is always three semitones to the left of the Major. Importantly for the purposes of using ReaTune, you might find it will work perfectly well doing its tuning when tuned to the Major key, even if if the song is (strictly speaking) in a Minor key. This is because the notes are the same (mostly; there are subtleties to this, but let's not go any more complicated right now). But if it doesn't work, try setting the key the Relative Minor in ReaTune. (There are actually multiple types of Minor key -- ReaTune doesn't distinguish though. I guess it's using "Minor Melodic". Anyway...)
So, without meaning to brag, I have never used key finding software in my life. I can find the key of a rock or pop song usually in less than a minute or two, and can usually be playing along with it in five. I don't say that to boast, just to show you that it's possible. And the technique I use? Well, I do what roughly what I just wrote for you above. In fact, this is learning your scales in old school music theory speak -- but let me tell you, scales are to music what times-tables are to mental arithmetic. I have an advntage over you in that I know all my scales without having to think about it (because I've learned them since I was a kid). They are etched into my consciousness. But I can't begin to describe how valuable for music production and performance knowing your scales is. Well worth a bit of effort.
HTH.
PS. I say etched into my head. However, if I am trying to play E flat Major my fingers often go off on a C# minor thing on their own for no reason at all. I dunno why. Stupid fingers. There's no controlling them...