In Dec 2017 I Bought bitcoin through coinbase haven't sold anything in 2017 but bought other currency from bitcoin in 2018, how do I account for that in my taxes?

For 2017 you do not enter anything on your taxes regarding the purchase.

You will enter this when you actually sell the bitcoin, and at that time will enter the purchase date, the price paid, sales date and price sold for, all on the same return in the year of the sale.