The best exchange to use would depend on some factors, like your locality, your trying currency; fiat or some other cryptocurrency and also your stand on identity verification.
You can purchase through binance; they have a limit on withdrawals that can be made without KYC.
Localbitcoins is also a good medium to purchase.
What's the difference between a stop and limit order?
A stop order could act as a means of preventing losses, if you enter into a trade, you can set a stop order to automatically pull out once it falls to a certain price.
A limit order is used when making purchases, if you do not want to use the market order, you set the price you want to buy or sell at, and once that value is met your order is filled automatically.
You should note that these concepts can have different applications in different contexts.