If you are not self-employed, your current annual salary (i.e. the annual salary of your current job) is what you should put for "my current individual annual income" (Part 6 #7), and if that is sufficient to meet the level, you shouldn't need a joint sponsor. You should include as many pay stubs from your new job as you can to prove current income, as well as an employer letter if possible.
Since you didn't file a tax return for some of the last 3 years because you weren't required to, per the instructions, for the question that asks about the "total income" from your tax returns from the last 3 years (Part 6 #24a-c), you should put "N/A", and then check the box in Part 6 #25, and attach a statement stating that you weren't required to file because your income was below the threshold for each of those years.
If you decide to use a joint sponsor, the joint sponsor would not appear on your I-864 form. They would fill out a second I-864 form, with their own household size and household income. Both I-864s (yours and the joint sponsor's) will be submitted.