Since there are two questions here, we'll answer them one at a time, in the interest of thoroughness.
Is Ancestry.com safe?
Yes, Ancestry.com is very safe! This is partially because it is a website that more-or-less requires you to pay to use it (though we will teach you some tricks for getting around that), so it's usually not worth the effort for troublemakers. Plus, the only people you're paying are those at Ancestry and their affiliate businesses, so there is little opportunity for any users to scam you. Though their staff may contact you from time to time, they will never ask for your account password, social security number, or any confidential billing information (at least without identifying unique information about something you've bought). And Ancestry's computers and software are designed so that it's perfectly safe to purchase things using your credit card.
The community on Ancestry is also mostly there to use the website properly; that is, to do genealogical research, and help others with their own projects. Still, to ensure your privacy while using the website, nobody is allowed to view living members of your family tree (even if your tree is publicly viewable), and nobody is allowed to edit your family tree or the records of anyone in it except you. The only exception is if you choose to give people these permissions when you directly share your family tree with them.
Of course, there are options to hide your family tree so that nobody else can search for it, or see it without your permission. You can also choose to block other members from contacting you. However, as mentioned above, most users are just there to work on their own genealogy projects, and some might be more than happy to help you with yours, so don't be shy!
Is Ancestry.com reliable?
This is somewhat of a complicated question, but we'll try to answer it as best as we can.
As far as the actual records on Ancestry.com go, they are mostly from official government sources, so it's safe to say that they are pretty reliable. Many of these records have been digitized (i.e. they have been copied from their physical paper or film formats into computer files) and can be displayed right on Ancestry itself, so you know that you're dealing with legitimate history.
There are, however, a couple of things to keep in mind. The first thing is that digitization technology is fairly new, and so it takes (and has taken) a significant amount of time for the company to find the different kinds of records that it stores, digitize them if necessary, and sort them into the online databases for Ancestry. Additionally, some records may not be legally available to the public yet, or some records may be too old to be recoverable (i.e. they have degraded over time or have otherwise been destroyed). In short, this means that the records related to your particular family may not be on Ancestry (at least for right now,) and you might have to look elsewhere for them (such as a library, archives, museum, etc.).
The other thing to keep in mind is that most of the records on Ancestry are from the United States. Though you can access records from other countries (including ones from the international affiliate websites of Ancestry, such as Ancestry.ca for Canada) with certain subscription packages (see our How Much Does Ancestry Cost article to find out which ones), it may be difficult to find records about ancestors that were born overseas.
Beyond that, though, how "reliable" Ancestry is depends largely on how skilled you are at genealogical research. Some people complain that the "Hint" system on Ancestry isn't all that accurate, though that is largely the result of not inputting enough information for Ancestry to make a better guess. The reality is that you can and often will find what you are searching for on Ancestry, if you know how to look for it. If you need some help, visit the "Learning Center" on Ancestry to get pointers for first-time genealogists, or go to the online support forums or message boards and see if another user can answer your questions. If worse comes to worse, you can even ask another user to directly help you with the research in building your family tree, or even ask an expert straight from Ancestry (though this may cost extra money)!