Many computer Operating Systems (OS) come with a program known as a photo editor.

The resizing function is usually accessed via the menu bar, tools option.

Apparently Windows 10 OS also comes with a drawing program, MS Paint.

One of the tools in Paint is a resizing option.

You have to open your image file in a program of that type and locate the resizing function.

Then you save (Save As) the resized image under a different filename, otherwise you will write over

the original large image. The smaller image is then suitable for upload to various websites.

For computer forums, an image width of 600 pixels is plenty (the resulting filesize will be much smaller).

The image height will vary depending on whether the image is landscape (wider in horizontal) or portrait

(taller vertically) view. Trying to upload and then render large image files on the Internet can take many seconds on

some devices. It is such a problem that very popular forums such as Pinterest and Houzz automatically

resize the images, sometimes with undesirable results. Even then, it can take awhile to upload

the large file. It is better to downsize the image beforehand.

See if you can locate a photo editor or drawing program on you computer and start from there.

I am also no tech wiz, but I sit with an Apple computer too. At least with the OS we have, (it's 10.13.6)  it's pretty easy to resize a picture. This is how I do it.

From the program that is now known as 'Photos' (this is where your pictures are kept when you plug into your computer, from a camera or a phone. I assume you probably have that figured out).

I drag a copy of a photo I select to the desktop, for resizing for the forum and to keep track of for ease of finding when I'm ready to post.    I open the image from the desktop copy, the default program it opens in is 'Preview'.   Select the 'Markup' button in the menu bar.   Another menu bar drops down, one button reads 'Adjust Size',  select that one.       The width and height boxes are exposed (in my latest; 3000 something  by 4000 something) I drop off the 1's decimal place in the width, the height decreases automatically, leaving me a 300 something by 400 something pixel photo that will upload to the forum quite nicely.    Select the ok button and close the resized photo.   When you're ready to post, select the altered desktop image from the photo upload menu . It looks quite small on the thumbnail but presents well on the forum. ....When I'm satisfied that every thing went through... I trash the desktop copy, leaving the original unedited in the 'Photos" library.  

Thank you, sweepbjames.  I also have an Apple computer and have never been able to upload pictures.  Your instructions worked, and I was able to post my first picture to the forum!