It’s very important to remember your seed phrase, so don’t lose it!
You can backup the wallet.dat file by making a copy of it on an external USB device. But if you transfer the wallet anywhere else for safe keeping, please make sure to secure those locations as well. An extra layer of security is always better than no security at all. 🙂
As for which exchange site (or multiple sites) to choose, research and find out where new tokens are launching first and/or someplace with good volume and low fees like Binance / Bitfinex / Kucoin . Once you’ve researched all of these exchanges, give Coinbase a try — we’re confident they’ll do the best job for you (though that may or may not be correct). It’s quite easy really: https://www.coinbase.com