First, we need to find the scale tool. I've put an arrow to it so it's clear

Select the scale tool, click on the image and then a box will appear with the current size of the image.

Click on the small link next to the image size to keep the proportion the same and then change the size to whatever size you want. Click Scale.

Now the image will be tiny if it was big in the first place, so go to the zoom (magnifying glass) and click.

Make a square around the small image with your mouse and then let go of the mouse button, it'll zoom automatically.

Now your image will be bigger to you, but also blurry.

Okay, so it's time to crop your image to the size you want. Use the crop tool to get rid of the chequered part at the back of the image by making a square around your image. The outside lines are where the image is going to be cut to. And if you look to the bottom left of your screen, you can check the size too.

Zoom out to see how it looks, and you have a blurry, but smaller image.

The blur is annoying, right? Okay, so go to Filters >> Enhance >> Sharpen.. and click!

Another mini box will come up, and you can play with the strength of the sharpen by moving the little button over the line, or by changing the number. When you're happy, click Ok.

Now your image should be ready for you to play around with as you wish