In terms of forms, you will generally receive the forms you need in the mail. These include
one or more W-2s (if you were an employee),
one or more 1099-MISCs (if you did contract work),
a 1095-A or 1095-B or 1095-C (proof of health insurance),
perhaps a 1099-INT (interest received) or
1099-DIV (dividends received)
although there are potentially dozens of forms, many don't apply to most taxpayers.
What is important on forms is that you wait until you get all of them before you file. There is nothing more frustrating to the taxpayer (and no fun for the IRS) when you get another W-2 the day after you filed. Remember that you file ONE tax return, no matter how many W-2s or other forms you get.
In terms of information, you need to know your filing status (are you married? single? separated?). The IRS definitions don't always correspond neatly to real life, so read the Section called "Filing Status" in IRS Publication 17 from pages 20 to 25.
You need to know if someone else will claim you as a dependent. This frequently catches first-time filers who get a job and pay taxes but don't realize that in the first year, they may still be dependents of their parents. This can affect education credits, HSA contributions, personal exemptions and so on. Read "Personal Exemptions and Dependents" on pages 25-37 on the link above.
For most first-time taxpayers, though, the process is easy - a W-2 and a 1095-C (to prove health insurance), and you e-file a 1040-EZ in literally just minutes.