To help your clients pay you by using their credit card, you need to have an account for:
PayPal
PayPal PayFlow
Authorize.net
Stripe
To set it up you need to click on your profile picture (bottom left corner) → Company settings → Project accounting → Online payment gateways and set up your credentials.
After the setup is completed, make sure to check the option Accept online payments when creating an invoice.
NOTE: Please be aware commissions will apply, for details please check your payment gateway providers page on commissions.
NOTE: this isn't available for US customers, they will need to use PM Payments