It is very important to get your Computer and Photoshop set up correctly as this will make things much easier further down the line.
The first to get right is the Colour Space which you work in. You can read a vast amount on the Web about this but basically you have three spaces to choose from.
1. sRGB IEC61966-2.1
2. Adobe RGB (1998)
Working Spaces: The working space specifies the working colour profile for each colour model (a colour profile defines how colours numeric values map to its visual appearance).
This will assist in uniformity through out your photographic process.
Adobe RGB (1998) provides a fairly large gamut (range) of RGB colours and is well suited for documents that will be converted to CMYK. Ideal for print production work with a broad range of colour. Ideally you should also set your Camera to the same colour space as you use on your computer.
sRGB IEC61966-2.1 is the default colour space for many pieces of equipment but it reflects the average colour space used on CRT displays. It is the ideal space for Web work which needs very little colour information. I suggest you give this one a miss.
ProPhotRGB this is the one that I prefer to use even though my camera can only use the first two spaces. This is because it is a very large RGB gamut and is particularly suited to higher end applications.
To change the Colour Space:
1. Edit > Colour Settings > Working Spaces > RGB > drop down.
The next step is to get the correct screen resolution for you set up.
1. Edit > Preferences > Units and Rulers > Screen Resolution.
To find out what your Screen Resolution is. Right click on an empty part of your screen. Open Screen Resolution and you size is shown. Use the one marked (Recommended)
Mine is 1920 x 1080 pixels. Now measure accurately the actual width of your screen from the edge of the tiny black line across the screen to the other tiny black line, the more accurate you are the better. Remember this is not the measurement that the salesman told you about as this is the diagonal measurement of the screen and is used to help sell the screen as it is the largest measurement but not your actual screen size.
Mine is 20.92 inches. I am old fashioned so I still use Inches.
Once you have the measurements you divide the Pixel width by the actual width in Inches. 1920 divided by 20.925 You get 91.8 which is the real pixel per Inch of your screen and this is what you set the Screen Resolution to.
Once you have done this every thing you see on you screen at 100% will be a real life size.
I'll give you one other change you should make and that is to change you Print Resolution (just above your Screen Resolution in the Preferences ) to the Pixels per Inch recommended by you Printer Manufacturer, mine is an Epson and its Print Resolution is 360 Pixels per Inch. you will need to Web search for this or contact your Manufacturer.
I hope this has helped, if you have any questions related to Photoshop please ask, you can get me by replying to the Forum or you can go to my WebSite: www.kimmartinphotographic.com