Capital Gain

I am a single head of household with very little income.  I bought NVIDIA in 2017. I'm thinking about selling some later in the year to diversify into an ETF that is also  tech, so sideways. Since my income is  below $40,000, will I have to pay Capital Gains and  how much can I sell before I've sold too much and will impose a Capital  Gains Tax?

Thank you,

Roo