So, you've never used a graphics program of any kind, I gather…
Do you understand that in order to fit your image to the three different aspect ratios you mention you're going to have to crop and discard part of your image in at least two of your three cases, possibly all three?
Either that or you're going to have to distort your image by either stretching it or squeezing it.
What are the pixel dimensions of the image you're starting out with (so many pixels wide by so many pixels high) ?
What are you going to do with it? If you're going to print it, at what size? Or is it just to be watched on a screen? It makes a difference as to whether you'll have to resample it.
In any case, this will involve opening your image in Photoshop, going to the Image menu and selecting the Image Size… menu item, which will bring up a dialog box into which you can input your desired values:
Next time, please tell us at least what exact version of the application you have. The illustrations in my previous post many not match what you have.
This is the Photoshop forum. I stress this just in case you have Adobe Photoshop Elements, which is NOT Photoshop; there is a separate, dedicate Adobe Photoshop Elements forum at:
(It seems odd that you'd start out with the very expensive full version of Photoshop and not be familiar with its absolute basics.)
Please read this FAQ for advice on how to ask your questions correctly for quicker and better answers:
If you do have Photoshop, you're going to have to take some classes at your local community college or else do a lot of reading on your and watch video tutorials.
Photoshop is a professional-level application that makes no apologies for its long and steep learning curve.