Exchanges in a general term can be called as the liquidity providers. They are the platforms where buyer can find the sellers bet and vise versa.
To buy from fiat you can buy it instantly buy through your credit card. If find the credit card think can kill your time while buying at certain price then you can buy stable coins like USDT and exchange them later to the coin you deserve to have.
This vary thing due to time at bank transfer or credit card fund transfer can be cut off if your holding stable coins in advance. Stable coins are the coins who's price is same as USD but are crypto currencies thus can be held in crypto wallet can instantly be exchanged to any other crypto without bank or card intervention.
There are many exchanges that help buying Example: Binance, Simplex, Indacoin, Changelly, CoinSwitch etc., I usually buy at CoinSwitch as I feel they charge too less on buying thus sharing their guide. It is upto you where you want to buy and store funds:
You can refer this article to buy Coins using Credit Card on CoinSwitch -https://support.coinswitch.co/a/solutions/categories/35000132267/folders/35000216282?lang=en&view=all
You can use this link to buy using Credit Card -
https://coinswitch.co/app/exchange?from=usd&to=btcNote: At any point of time please make sure that the funds are in your wallet and you are not sharing your private key with anyone.