Load test web application API/database

I´m assigned as tester for a web application and is quite new in web development world. We will develop a web portal (.NET) for users, this portal will be fed with credit card transaction information from two ways. Some will be transferred through web services and some will come through SFTP. How can I perform load test on these APIs and database?

WS(data) ----> [Web portal (DB)] <----- (data)SFTP

Thankful for any help!