Hi! Hopefully I can describe this in a simple way that can be understood - but not knowing what photo viewing system you are using to view the pictures, I'm going to have to keep the description generic (if this doesn't work, just post back exactly what program your computer uses when you view your pictures).
First, you'll want to keep your photos as they came out of the camera, at the largest possible size, for your archives. When you resize a photo, you definately compromise the resolution and ability to make prints from it...so you never want to resize your one and only copy. Usually, the best idea is to make a second copy of every picture which is smaller, that can be used for slideshows, posting to the internet, etc. The original is always still there if you need it.
To resize a photo, most photo viewers have a 'resize' function in the menu - either on the drop-down toolbars, or on an icon in the toolbar. Look around for this 'resize' function button or command (if it's in the menu, it's usually under 'edit' or 'tools'). A very common and good size to reduce your photos to for posting is 800x600...this will either be a pre-selected setting, or you can type it into the resize box. Your photos will come out of your camera usually quite a bit larger than that (a 3MP camera might shoot at 1600x1200, a 4MP camera at 2300x1700, a 5Mp camera at 2560x1920, 7MP at 3072x2304, etc)...so reducing it to 800x600 will make it smaller and easier to post and view on your screen without having to scroll around. Once you've entered to resize the photo to 800x600, click OK and the photo should resize itself.
Here's the important part - so as not to compromise your original photo by saving over it with the new small version...you need to save this copy as a new file. To do this, go to the 'FILE' command at the top of the menu bar, and scroll down to 'SAVE AS' instead of 'save'. This will bring up a Save box which will probably already be defaulted to show the folder you are already in, and with the current picture's original name already pre-typed in the name field. You simply have to give the picture a different name than the one it originally had - for example, if the picture was called 'dsc-0001.jpg', you could rename the smaller version 'dsc-0001s.jpg', with 's' denoting the 'smaller' size. When you save the picture that way, it will add the smaller copy of the picture to the album next to your larger original, without causing any damage or loss of resolution to the original.
If you have any problems following this, please give some more details about what type of computer you have, what operating system you are using, and what program your computer uses to view JPEG files.
Good luck!