Select the altcoin you want to buy, then signup on the existing exchanger market altcoin with USD. You can buy it there with your USD
Not only registering in the exchange at the same time you need to find the best altcoin which will make you more money. Don't be a rush in buying altcoin because you have to be very careful in choosing them. In the market, everything seems to be perfect but you need to make very careful steps in choosing them.