I know this was a while ago but due to changes in terms of tax revenue as well as internal revisions, Valve, which is located in Washington, is required to comply with their tax obligations as stated by the Internal Revenue Service.
They linked you to an FAQ but the really important gems are here.
https://support.steampowered.com/kb_article.php?ref=6088-UDXM-7214#addldocsemail
https://support.steampowered.com/kb_article.php?ref=6088-UDXM-7214#taxes1099
https://support.steampowered.com/kb_article.php?ref=6088-UDXM-7214#incometaxes
https://support.steampowered.com/kb_article.php?ref=6088-UDXM-7214#VATtaxes
Unless you manage to find yourself beyond those thresholds, there is nothing to fear. If you are not a United States citizen, like me, then Valve will ask for information that they can give to the IRS proving this. What this will do to us however is unknown, it is simply a means of being compliant with the law. As usual, that information you provide will not be used for anything else and I believe will only be retained when, if, the federal agency asks for it.
I hear that the IRS is having a field day this year for 2014. The minimum amount required to 'report' on certain individuals rose quite high, resulting in more Americans being owed money instead of vice-versa. In fact, there was an article about it not too long ago.
http://abcnews.go.com/Business/unclaimed-irs-tax-refunds-lost-cash/story?id=23012062
You have to admire the naysayers on this one. It is hilarious, they still portray the agency as an evil abomination that turns you upside down and takes your hard earned cash in the name of 'Uncle Sam', yet will put that money back in your pockets if you fall under the criteria for it.
Also should add if you ever do get a call from anyone from the 'IRS', hang up. Should they ever need to get a hold of anyone, they have their means and it is usually through the U.S. Postal Service.
http://www.irs.gov/
I am just worried because I don't want to fill out the form without my parent's permission.. Do you have to pay if you are not a U.S citizen?
No, again unless you somehow exceed into the big dollars, like you see some names thrown about about being 'Pro-Traders'. However, unless you aren't of legal age to even BE on Steam, you should reply as soon as you can to avoid any potential problems coming about. They could restrict your sales until you assist them with this. You should not withhold any information from them however, when you provide it. Falsely giving them information...don't hold me to this but could actually be a crime, as it is given to the IRS and if they reject it, you would not want some angry fed banging on your door.