I'm trying to create a kitchen visualization and I want to switch a current photo of a marble backsplash against an arched plaster surround. How do I fit an image of my marble into the image so that it's the correct shape.

Put your marble in a Smart object, transform it to size and use a mask to get the shape. The easiest was to create a mask that shape is to use then pen tool to draw a path then create a mask from that path. Once done, you will need to add shadows to make it look realistic.
Dave

Welcome! The classic way to do this is a layer mask plus a warp.

First, place your marble photo as its own layer above the kitchen photo: File > Place Embedded, then press Enter to confirm. Next, select the existing backsplash area — the Object Selection tool is the fastest for this, or the Pen tool if you want a precise outline of the arch — then, with the marble layer active, click the Add layer mask icon at the bottom of the Layers panel, so the marble shows only inside that shape. Finally, choose Edit > Transform > Warp and drag the grid handles so the marble follows the arched plaster surround; dropping the marble layer's opacity to about 50% while you warp makes it easier to line up, then set it back to 100%.

Adobe's help covers both steps: adding layer masks (https://helpx.adobe.com/photoshop/desktop/create-masks/layer-masks/add-layer-masks.html) and the Transform > Warp options (https://helpx.adobe.com/photoshop/desktop/crop-resize-transform/transform-manipulate-reshape/transformation-options-in-adobe-photoshop.html).