I forgot to claim my Uber income from 2014 and now I've got a tax due bill, what do I do

If you agree with the tax owed - I'd expect the IRS caught this because they received a 1099 from Uber and used that dollar number in their own calculation of additional tax due - then you can simply pay the tax, (plus any penalties and interest), and that should be the end of it.

However, I'd also expect that the IRS-calculated tax is too high because they are only taking into the account your "revenue" from your Uber activities and not factoring in any "expenses", which you surely have.  I'd advise purchasing the 2014 TurboTax program - maybe "Deluxe" to start and then upgrading to "Home & Business" if you feel you must - and amending your "as filed" income tax return, reporting both your income and your expenses.  That should result in a tax due that's less that the IRS proposed.  You'll need to buy a desktop version of TurboTax as a program for only one tax year is available online and that's 2015, soon to change to 2016.

This TurboTax FAQ - https://ttlc.intuit.com/questions/2589210-how-do-i-amend-change-or-correct-a-2014-return-in-turbotax - will get you started on the amend process.