What is pay per install ?

Ok, first of all pay per install or PPI is not another money making scheme that will get you filthy rich over night, PPI is BlackHat method and you should know this from start, it is Blackhat and it may be illegal in some countries, you should be well informed before starting with this business.
I will just try to explain to you what is this all about, and you decide are you right for this kind of job or not.
Remember, this is not some scam affiliate BS where you don't have to move your finger, you have to work, risk, be creative and know how this god damn thing called internet works, in other words you will need some social engineering.

So you have those PPI companies that offer you money if you get people to install application that they provide to you, they give you around 0.10$ to 0.50$ per install, some give you more some less. Money you get per install depends in witch country your install got, you usual have list of high paying countries on companies web site.

How do you get users to install your application ?

You should read TOS of company you are registered to, but usual they allow all kinds of marketing your app, from P2P and torrent, to any "sneaky" method you can think off.
So you don't need a web site with a lot of visitors, you don't need web site at all.
Of course the standard method is to bind your PPI.exe (application you got from your company) and another application you would like to share for people to download AND install.
So when people download your program they will get application they wanted to download but with a little extra thing, your payload and BUM! you got yourself an 0.50 cents just like that :)

There are a few application that will do the binding two or more files but you have standard one in windows called iexpress, but those better binders are not free.

What is that application that they have to install ?

It is Adware, YES it's that thing you probably hate, but it will bring you money for sure, you will forget that you are spreading adware as soon as first check arrive. Different companies have different adware, some install toolbars, other change hompage of browsers etc.

How much can i earn ?

If there is such thing as stupid question then its this one, people allways ask how much they can earn but never think by themself. Peoples earning depends on all sort of things, main thing is your method of spreading file, if you find some good idea then you will be earning more with less effort and so on.
If you use torrents, file that you upload will be earning you money for 4-5 days, then it will probably get deleted, but it dosen't need to be, i heard people have torrents up for long, long time and some of them are in top 100 on TPB. So there are no rules in this buisiness, only the ones you think of, imagination and social engineering should be everyones goal to earning more and more.
From the proof of earning that i saw, people usual get from 10$ to 15$ a day after one week, after that it all depends on you, but it could never be less if you work 2-3 hours a day.

Can i get scamed ?

Well, there is no buisnisse where you can't get scamed, but if you stick to the well known PPI companies then you have nothing to worry, they earn a lot of money from you and they have no reason to scam you.
Some can "shave" your isntalls, meaning they will show less then you actual have, but that can be fixed with a little code from you, but remember they need you and you need them, they will try to help you as much as they can to earn more for you and them.
And don't try to cheat on them, it is allmost impossibile, you will only earn a ban.