The simplest way I know to invest in Bitcoin is still the same way that I first invested in Bitcoin. I went to coinbase.com, created an account, verified my identity, connected my bank account, and bought a fraction of a Bitcoin in exchange for its equivalent value in dollars plus a small fee. It is that easy. If you want to invest in Bitcoin right now, you may do that and read no further.
There are many arguably better ways to acquire Bitcoin and certainly several better ways to hold it than on Coinbase where they are holding it on your behalf like a bank account does your fiat currency. Nevertheless, unless you are ultra-wealthy, investing and holding 1% of your money in Bitcoin by these means at least for the time being is in my opinion a reasonable and responsible way to go about it. If you are ultra-wealthy or want to invest more than a few percentage points of your money in Bitcoin, I would recommend considering alternative ways to hold your Bitcoin which are a bit less seamless to setup but are technically superior in an analogues way to how having your valuables in a safe is technically superior to having your valuables in a safety deposit box at a bank where, for example, under certain circumstances the government could force your bank to forfeit your valuables to them for some reason or another. These are circumstances that are important to think about for Bitcoin in the long-term as the system could find enemies in governments and alike, but for a low percentage of your money at this point in time, and most likely for at least the next few years, I see relatively little risk in initiating your investment in Bitcoin this way.