I’d recommend you get a book or type of study guide. There is a great guide on here that @Lady_Eva put together:
Simple English Demonic Evocation Guide
Also, if you need general reference to find the right spirit for the results you’re looking for check this out:
Finally since you’re starting out, from my experience, I’d recommend you buy a copy of Demons of Magick by Gordon Winterfield. It’s a great reference and has methods I’ve found to be effective.
Feel free to reach out to me or ask questions in here if you’d like. If you follow the links I copied in this post, you should be able to learn a lot of information.
Best of luck.