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:
