Trading 101.
You buy something you believe will increase in price and you sell when you believe something is about to fall in price.
On exchanges there will always be a trading pair with BTC and your altcoin of choice. There might also be other trading pairs not including BTC but those probably vary with different exchanges, Bittrex also has ETH as a base currency.
Whether or not you trade BTC for dollars is dependent on if you believe one will rise/fall in regard to the other. As long as you're on the right side of the trade, the currencies doesn't really matter. To you they are just numbers on a screen and you want your numbers to increase.
If you want to cash out, you cash out.
I'm not a serious trader myself, a n0ob if anything, but so far I've been doing ok. Just take it one step at a time.