Of the recommended gateways, Braintree has the most widespread support around the world. To start getting set up with Braintree:
1) Apply for a production account by going to https://apply.braintreegateway.com and fill out the information requested
2) Because it can take some time for Braintree to review your application, you can also start by going to https://www.braintreepayments.com/get-started to create a test marketplace and gather the keys you need to start testing transactions.
3) After applying for a sandbox environment, you will be dropped into your Braintree dashboard immediately, and can gather the API keys that you need to start testing Braintree
4) For more information on how to gather the API keys that you need for successful integration, please visit https://articles.braintreepayments.com/control-panel/important-gateway-credentials
5) Once you have the keys for either your test or production environments, you can navigate back to your admin dashboard and plug those keys into the appropriate fields.
6) You can then enter test mode and test out the test credit cards laid out here: https://developers.braintreepayments.com/ios+dotnet/reference/general/testing.