What is your experience in Photoshop? Do you have any training? This issue is just a matter of using layers. The original size for the document should include the size of the image and space for the text. If the proportions of the image don't fit exactly, not a problem. You just adjust the surrounding "white space" to fit everything or crop the image.
The bottom layer will be your background color. One layer above that will be your image and another layer will be your text. When you're done resize the image to your needed specifications in Image>image size.
If you want, you can post your image and the corresponding text and we can give you further direction. But it is extremely important that you have basic skills in PS and understanding of its function.
If you do not, then I suggest you do some basic tutorials to get trained in the software. It is a complicated one but the basics for what you need to do are quite simplistic. Why spend hundreds of dollars on Photoshop if you don't intend to learn it? Or perhaps you only spent $50 or 60 ... JK ... maybe ...
