In need of a little guidance and inspiration for a content box's background

Hello! I am making myself a website and am in need of a little inspiration and guidance. I am basing my design on the attached file and I would like to make a similar background for the topmost content box. I would like to know if any of you guys know any tutorials teaching how to make a background with those highlighted "waves". I tried making curves with the pen tool, closing it into a box, making a selection, filling it with white, setting the opacity and fading it by erasing the borders or masking, but it wasn't a very good result.

I am also interested in any tutorials that teach how to make other professional looking backgrounds or any websites that are similarly structured and could serve as an inspiration. Just for reference, what I intend to put there is a similar dialog box where the person chooses what type of service they are interested in from a menu, such as "Translating, Proof-reading, Subtitling", etc. Any other tips are welcome.

Thank you very much!

I used to go to a this one website to learn a lot of different techniques for web design and all. The website was called Photoshop Tutorials, Flash Tutorials and More! P2L Tutorial Search hope it can help you out.