Hi @Diga ,
Here are some videos to help you get started with the InvokeHttp Processor:
https://www.youtube.com/watch?v=aNrdWYy4i34
https://www.youtube.com/watch?v=SZhX_gce63E
https://www.youtube.com/watch?v=Jk7H8w3evN0
As far as logging in and authentication, it depends what kind of authentication the Rest API has. If it has simple authentication (username & password) you can enter those in the Basic Authentication Username & Basic Authentication Password properties. If you need to get Access Token (OAuth2) then you either obtain this through another InvokeHttp processor and pass it to the actual rest api invokehttp as "Authorization" header property (dynamically added to the processor), or second option is to take advantage of the "OAuth2 Access Token provider" in the InvokeHttp processor by setting up "StandardOauth2AccessTokenProvider" service and provide the credentials and the access token url there.
Hope that helps.
Thanks