I agree with the advice given, do a search for some online tutorials- Adobe themselves have a great list here
and a huge You Tube list here
. It really is an amzing bit of software, and can seem very hard to get your head around.
One of the best ways to learn it is to play with it. Make a copy of your image- never work on an original- and just try out different tools/effects on it. You can create files on your pc, store the altered image, with notes about how you did it.
I assume most of the features/controls are the same as Photoshop itself, in which case, re-sizing an image:
Go to File: Open, and select an image.
Now, click on IMAGE, go down drop down box to DUPLICATE. Click that.
You will get a pop up box that says Duplicate Image, and gives it a name. Click OK. now have your original on the screen, plus a copy. Close the original.( Click on the little x).
Now pull your image away from where its attached to the top of the screen. Just put your mouse on the tab and drag.
Click on IMAGE again. You get a drop down list.
Go to Image Size.
You get a box, with an assortment of options.
At the top is width & height, the default showing is Pixels. It also tells you the size of the image in megabytes- the one I have open is 5.49M, this is useful to know when you are uploading to some galleries that have a size maximum. You can see in the second image I have made the image size smaller by typing in a lower number, and it has also made the pixel dimension, and therefore megabyte size smaller.
Below that is Document size, in width, height & resolution.
Default showing is centimeters. If you click the arrow next to that, you get a drop down list of choices. I always swap it to inches.Work with what you understand best. Now you can see the size of your image in inches. Here is where you alter the size of your image- just change the numbers in the box.Click on them to highlight, then alter to what size you want. Don't forget the decimal point, lol. i.e, 1.5= one and a half inches.
Next to the width/height boxes, you will see a 'linked' icon- this means these two dimensions are linked by proportion, so when you alter one dimension, the other will automatically alter in proportion, keeping your image looking correct. You can change this, if you look below this box, you will see three tick boxes, Scale style, Constrain Proportion and Resample image. If you untick Constrain Proportions, you will see the linked icon vanish, and you can just change one dimension. It will make your image look stretched, or squashed though.
Back in the document size box, you see Resolution, in Pixels per inch, or centimeter. The higher the number of PPI, the bigger the image will get- usually without it going pixellated.
At the bottom, Resample Image has a drop down list if you click the arrow. The ones to use generally are Bicubic Smoother( best for enlargement) or Bicubic Sharper( Best for reduction)
Hope this helps a little. Just remember, as long as you work on a copy, you can't destroy anything of value, and if you click on EDIT, after anything you have done, you can UNDO, or Step Backwards, to remove any change you made.