I've spent hours in the past few days, looking at online family trees, for several ancestors, that have so many errors and misdirections.... I can't tell you now annoying it is to waste so much time verifying that things are incorrect and trying to find the correct information.
Here are a couple of hints that may make you better researchers (you can be sure I'm not the best but far from the worst.):
Who - as complete a name, including initials, as you can find. Write down any spelling variations you have. Make a page for each individual.
What - what are you trying to find? Do you want to know where they were born, where their family was originally from? Try not to be distracted from your goal, it's easy to find yourself off on a tangent, chasing a related piece of information. Don't go there.
When - write down dates, as precisely as you can. Birthdates, marriage dates, death dates, children's birthdays.
Where - write down places, birth places, wedding places, death places, burial places.
Check dates and places. If you have a birth date and a marriage date but they are farther apart than 20 or 30 years, you may have an error somewhere. Most people back in the 1800s and even now, get married in their mid-20s. There will be exceptions but this is still something you can rely on. They also had children right away, usually.
DON'T JUMP TO CONCLUSIONS.
If you have an ancestor who was born in county X and died in county X is said by somebody else to have married and lived in county B, unless you can verify it's the same person, it's unlikely that it is.
You WILL find more than one person with the same exact name as your ancestor and often nearby where you ancestors lived: Who would think that there'd be more than one Charles and Francis V. Dougherty in New York State, but there were and the couple who lived in Buffalo, were NOT my relatives and so many people with the same family line seem to think so. For a while I had an ancestor who I thought must have been a bigamist because I saw him in 2 places with different wives but children with all the same names, until I sorted out that they were cousins one a year or 2 different in ages who, unhappily, named their kids almost exactly the same names.
A lot of children died. It's something you will find and have to accept. People disappear from the family tree.
You will not find documents for each and every person but you should try to find documentation of births, marriages and deaths. Getting a real copy of a certificate of any of them is getting more and more expensive; the last I ordered was $22. I'm happy to share for the cost of a photocopy and postage - after all they're not only my ancestors and relatives.
You can expect errors on almost any record. It's surprising at first and really unfortunate but all these records were created by people who, just like us, make mistakes. Spelling is the biggest mistake but there are some real surprises. I have a death certificate of a great grandmother, the information supplied by an adult son who lived with her past his 40th birthday, and my great grandfather's first name is incorrect on the death certificate, unless we don't know something my great uncle knew. I choose to ignore the mistake since I have other documentation of their marriage.
Slow down. I have to tell myself this all the time. And,
Think. What's logical? What seems to be the normal progression of a person's life in time and space and that's the direction you should look.
Write down the source of every bit of information you find. Immediately. I have to tell myself this even more often and I often fail to do it and I'm ALWAYS sorry later, if I don't.
DO NOT accept family information from relatives as facts; sometimes they are, sometimes they're not. Information gets handed down, generation to generation, cousins to cousins, grandparents to grandchildren, and like the kids' game of telegraph (you know where someone whispers something in the ear of one kid who does to the same to the next kid on a line however many kids long and the last kid in the line says what he was told and it's almost always completely different from what was whispered to the first kid), the information gets distorted over time until it's as far from the facts as can be. Usually family information is a good starting place but it's better if backed up with a family bible, or birth certificates, funeral cards, etc.
Most of your ancestors and relatives, particularly in the mid-1800s, stayed close to home and, when they relocated, they often did so with most of their family. So, if you see someone with the same name over the border in another state, or 3 counties away, don't assume they are related until you have checked carefully, all the dates, names, etc.
Follow your ancestor from birth to death to get a clear picture of the flow of their life. Say you have your great grandmother's birth date and her wedding date but not a clear place for either. If you know her birth date and her maiden name, then you can find her on the next census after her birth. If you don't know her maiden name, but you know her wedding date, you can find her on the next census, after her wedding, with her married name, or your great grandfather's name.
FamilySearch.org has most of the census records and many birth, death and marriage records - FREE.
I'm a paying member of Ancestry.com, so I don't know which of their records are free now.
Finally, please, Please, PLEASE, don't post your information online unless and until you have documentation of what you're sharing. Or, at least, include notes indicating what is and isn't verified.
I now have a clue that an ancestor that everybody else has accepted as one of my 4th great grandfather's father, is somehow related but not related as everybody else thinks because the dates and places just don't match. I'm more than a little tired of wasting my time looking at bad information and arguing with people who either don't understand how important it is to get things straight or don't care - like somebody very well-known is about everything that comes out of his mouth.
And, yes, I make mistakes but I work on my family lines to try to be sure that I have documents verifying who is who and what is what.
After all, you can believe that you're related to Warren Buffet and tell as many people as you want but I'd be careful if I were you about showing up for a reading of the will when the time comes, unless you have more than one document proving it.