The very best are primary sources: birth, baptism, marriage, death and burial certificates, census returns, naturalisation certificates, wills and personal letters written by the person concerned etc. Oh, perfect world. After that come collections of material, authorative biographies, newspaper reports. "Told to me by my grandmother on such and such occasion" if it refers to something she experienced is good as is a report of anything you witnessed provided you add that you witnessed it. You'll probably have to put up with something less than that sometimes. User created Family Trees are not considered good sources as you cannot tell how good the author is. If they says where they obtained their information it is better. Family Bibles etc are considered much more reliable especially if they were written at the time the events happened. You can use sources that others cannot easily see (books you own or websites people have to pay to see) but it is best if you report exactly what was said, for books add author, ISBN etc. Great that you asked this at the beginning. Enjoy.