Ledger doesn't provide any exchange services. You need to buy bitcoins from an exchange platform or from anyone owning bitcoins and who is ready to sell them. We recommend you to fully read this website to know how to start with bitcoins, and before going further: https://bitcoin.org/en/getting-started
To send your bitcoins to a Ledger device, you will just have to:
- Copy a receive address managed by your Ledger.
- Paste it on the exchange service from where you will send the bitcoins.
Please consult this article if you want to send the bitcoins you have purchased on your Ledger device.