The information Paul shared is excellent. I would like to add a couple of things.
If your shares were purchased through an employee stock purchase program, then this resource may be helpful to you.
The important thing to note is whether you have a 'qualifying disposition'. This means that you sold the shares at least 1 year after the purchase date AND 2 years after the offer date. In this case you will report your 'discount' off the market price as ordinary income, and the gain in value on the shares as long term capital gain. Your employer may or may not have included the discount on your W-2, so check carefully to ensure you don't double report it.
If is not the case, then your employer will include your 'discount' on your W2 as income, so do not double report it. The gain may be short or long term depending on your situation.
The resource Paul shared will help you determine which capital gains rate applies to you so you can estimate your tax due.
Be sure to add the 'discount' amount (which was already taxed as ordinary income) to your basis so you don't report too much gain and get double taxed.
