Test form username and password, method POST, REST api, SoapUI

Hello, is there any way how to test user name and password for web site form. Lets say user willl insert correct username and correct password to form, the test assertion will pass on the other hand assertion wil fail. Any help how to do it-process or any easy video tutorial link? I dont know where to put username and password and dont know how to create teststep with assertion:catsad: Thank a lot a lot for help.