Some important tips and Tricks.

(is it "dangerous" for my PC or my pocket?)
No, it is not dangerous, if you know how to protect yourself (and your PC).

Do not invest money (from your own pocket) in PTC sites
If the site you have invested will turn into a scam after a while you will lose your money. Be patient and also be an active clicker. After three to six months you will have enough money to buy referrals and you will also know which site pays and which is a scam.
If you can't wait and you want to invest for first time I suggest spending up to $30-40. You'll buy 15-35 referrals and get your money back within 1-2 months. If you ask me what I would recommend: just be patient...
So, do you want to invest?

Do a google search
Before you register a new account in a PTC site do a google search about the site you want to become member. Google search phrase: Company name + scam (for example bux.to scam)

Referrals
If you decide to buy thousands of referrals, prefer to "split" them in many different sites. Instead of buying, for example, 2000 referrals in one PTC site, buy 500 in four PTC sites. If you have bought referrals do not expect all of them to be active. Usually you will receive 2-3 clicks per day per referral. And remember that it is possible to receive bots and not humans.


Do not cheat
Do not try to cheat (multiple accounts or autoclickers). Probably you will see your account to be banned. Autoclickers also may contain viruses.

Black lists
You can take a look at some scam/black lists to see if a PTC site is scam or not: Paytoclick Blacklist and also check TalkPTC Scam list.

Something else?
Read very carefully the TOS (Terms Of Service), you may find useful information. Some sites for example do not pay; they only are ads exchange sites. Also read the FAQ section. You may find useful information there too.
Don't "blind" click. In several sites there are some ads with title "anti bot detector", "anti cheat link" etc. If you click an ad like this one your account will be deleted and you'll lose all your earnings. These ads are there for auto-clickers and users who do not speak english. So, be careful!

Don't spam. No one likes spam. Especially PTC owners. That's why they ban spammers from their sites. But why PTC owners have strict policy about spammers? Because sometimes users who receive spam mails (e.g here is a great site... and a ref link) are reporting to the providers/hosts the sites and as a result the domain of the site is suspended.
Personal data. Some sites are violating their own terms of service during registration. Sites, which have a term like this: "You are not an employee of [site] and you are not an independent contractor for [site]. We simply create a middle-man connection from the advertiser to the consumer." At the same time are asking for personal data like full name, address, city, phone number and zip code. Such sites must be avoided.
Read forums. Reading forums can help you to find if a site works properly (=pays) or not. Complaints, success stories, active/inactive admin etc can help you to decide if a specific site is good or not.