You'll need to buy IOTA with either fiat currency or cryptocurrency. Use one of the exchanges linked below.
Completely new to cryptocurrency?
If so, it might be smart to keep your IOTA on an exchange until you learn more. The risks of keeping you balance on an exchange are well-documented, but the IOTA user experience is notoriously poor right now (still under development). You'll have to weigh the risks for yourself.
Comfortable with technology, and have previous experience with cryptocurrency?
Given that you have the appropriate technical knowledge, it's safest to keep your balance in a personal wallet rather than on an exchange. Download the wallet & send a transaction from the exchange to your wallet.