"I just started working... I can't provide my last year's taxes, only an employment letter and an estimated annual salary. Is it enough for my affidavit of support?"
You are required to provide your most recent tax returns (ideally transcripts). If you didn't file taxes, you'll need to provide at least evidence that you are exempt from filing taxes. Otherwise you'll need to file past tax returns and at least set up a payment plan with the IRS for the penalties and fees because, if you just ignore and pretend last year didn't exist, you're just inviting a multi-month delay and a request for evidence, which is what we usually see if you're not cooperating with what is required by the government. You have to provide this if you want to get approved and if you want to go through the process as smoothly as possible.