I want to select part of a photo I'm editting in Photoshop. This area is exactly 340 x 210.
How do I do that with say the select tool?
If you want to just select an area click on the marquee tool in the toolbar. Once selected, you'll see a drop down at the top called Style with the default set to Normal. Select Fixed Size from the option and enter the values into the right boxes. Notice, with this option, dragging the mouse is no longer necessary as the box is a fixed size. You just need to select the origin or the top left of the box.
A similar method can be used if cropping by entering the values in the boxes when the crop tool is selected. When cropping, it is possible to drag the box but it will stay at a fixed ratio just like the Fixed Ratio option of the marquee tool. This method differs in that it will resize the cropped area according to your entered values.
Are you trying to crop the picture to that area? What I like to do is set the canvas size (Image > Canvas Size) to your desired dimensions, and then move the layer around until you land on the desired area. Make sure your layer is not locked, I usually just Select All, Copy, Paste, and Delete the original layer, before adjusting the canvas size.