the answer to your first question is to look at the domain name, credible domain names would be: .edu, .gov, .com. You could also look at the date of the information, if it is outdated then it's probably not gonna be good info obviously because the new information could come up that trumps the old info. Lastly, you could look at the author of the info, if the author is looking to gain anything financially then it's untrustworthy, you should also look to see if the author has any merit such as a Ph.D. or recommendations.
The answer to your next question is because you wouldn't want your results to be too broad or too narrow so you would want to choose your words wisely.
finally the answer your last question, informational text is text whose sole purpose is to inform the reader about natural and social ongoings
Hope this more than answers your question :)
