Back Taxes- As a US citizen (and sponsor of an immigrant), you must have filed taxes in the US for the past 3 years (if your income was above the reporting threshold). As you will be the primary sponsor of your spouse, this is required as supporting evidence for an I-864, Affidavit of Support. If your income was below the filing threshold, you can explain why you were not required to file taxes. Be aware that US citizens must report world-wide income regardless of where you reside and regardless from where the income was earned.
Evidence that you intend to establish domicile & relocate to the US- You will have to provide evidence that you either reside or intend to reside in the US.
Sufficient income or assets to sponsor a new immigrant- As the primary sponsor of your spouse, you or a Joint Sponsor will have to show evidence that you have sufficient current annual income continuing after relocation to the US or sufficient assets to sponsor a new immigrant. This will be supporting evidence for the I-864, Affidavit of Support.