If I invested 30k in Ethereum and when my investment goes up to 60k, and I sell 30k just to get my initial investment back, will I be taxed on that? I live in the United States and just bought Ethereum this year.
No, there is no tax for you when you just sell the capital amount and continue to hold the gains.
But you should report your conversion and mention it as converting the capital not the gains and you will have benefits on gains if you manage it to hold the investment over a year to reduce the tax rate into half.