Entering into the crypto trading world is really something daring considering that it is unregulated therefore you might see huge gains and losses each day. Unlike the stock trading where a stock can get a buyer freeze or seller freeze if it is highly volatile there is no such freezing here we see coins increasing by 100% daily and a few falling by the same margin.

So answers to your questions. First of all if you are fiat holder get your fiat converted into BTC using coinbase or any other local exchange which allows you to do so. Its different for different currencies moreover USD deposits are widely acceptable across various exchanges too. If you are into daily trading I would recommend you to go with Bittrex. It has the most amount of coins listed with great versatility so you are always on trade. You can get easy withdrawals upto 1 BTC daily but do make sure you get some verifications done. Binance is a good exchange too considering they allow you to withdraw upto 2 BTC daily. Just make sure you are patient while doing trade and don't keep your high amount of funds on exchanges they are most prone to scams. Shift the funds to cold storage if you don't want to trade on them.