If I just answered your questions directly, you'd only get "wrong answers".
1) You ask for websites to open wallets, but my suggestion is to keep reading/studying before opening wallets, because if you do it wrong, then you will get your money stolen. There are websites that you can use to create wallets, but many of those are outright scams or vulnerable to attacks. The main problem with website-based wallets is that when you use those, you don't control your private keys. You need to study more, and most importantly you need to learn the idea about controlling your own private keys. Keyword here is COLD STORAGE (it's a technique you don't really need if you are are only mining and trading (and not hodling), but it's crucial you understand it anyway!).
Also, if you trade with an exchange, then they will handle the "wallet stuffs" for you, while you trade, but still, you must know how to withdraw your coins from an exchange into your own wallet.
2) While trading, keep your trading coins in an exchange. Once you're done trading with them, put them in cold storage (paper wallet, encrypted usb-stick, etc) or whatever wallet software if you want to spend them (making purchases on webstores that take payments in bitcoin, etc).
3) Exchanges only allow trading between certain pairs. Most altcoins have a BTC and ETH pairs, sometimes LTC or even DOGE pairs. This just means that you can't directly trade between say ADA and MED, but first you must trade ADA for ETH and then ETH for MED. In general, if you are uncertain about something, read the FAQ-files of the exchanges and then test with small amounts first.
4) You don't need to use BITTREX, it's only recommended because that way you save on some BTC tx-fees.
5) You should study USDT well before using it. It will only be "safe" for a certain while, after which it will lose it's value. Also, on a more philosophical note, USDT it a transition tool to help make the transition from a world where money is controlled by governments into a world that is decentralized and where value is transferred by crypto and goods like petroleum and food directly. It is my opinion that once you are "done trading", you put your coins in coins you believe in, coins you believe you will want to use in the future, maybe BTC or ETH. To put your money into dollars or USDT is weakness of faith, imo. You should only cash out crrypto into fiat if you are hungry and need to buy food, or if you want to buy a house, or whatever that you can't buy with crypto yet.
IN A NUTSHELL:
While trading, the exchanges will handle the walleting for you, and you only need to store the password for that exchange to access them. Once you are done trading, you withdraw your coins from the exchange. If you withdraw to hodl, you withdraw to COLD STORAGE. If you withdraw to spend, you withdraw to a HOT WALLET (different coins have different wallet softwares, and some wallet software can handle more than one coin (Coinomi on Android for example)).
Sorry for saying "you should study" so many times
I hope you will have much gains and many coins in your future!
