the easiest in the US is probably coinbase. Alternatively, if you have one set up, you could use a legacy brokerage and buy an ETF temporarily, and then convert for long term at your leisure. Some people do this with the silver/gold etfs when the spot price deviates hugely from physical price with the expectation of it reconverting, but also to "lock in" a price. with crypto you should not keep it on an exchange long term. you purchase it, and then when you are done trading you move it to your wallet that you control. There are many wallets and you will just have to do research to find what is best for you.