To use bitcoins, at some point you actually have to acquire some bitcoins. Unfortunately, doing so is not quite as easy as sticking a card into an ATM. The following are a few of the ways you can get your hands on some bitcoins.
Establish that the platform you are using or the person you are buying from is legitimate, as you would with any other online transaction.
Use an exchange such as Gemini to purchase. You register your details through this trusted exchange, deposit your local currency such as USD or GBP, and then purchase the bitcoin at the current rate of exchange.
Buy in person by purchasing directly in the same way.
Remember that once you have purchased your bitcoin, move them to a location that is in within your control. Don't store them long-term on an exchange.