My iMac is acting strange

1. I type a sentence and it becomes highlighted without me doing it and if I continue typing the highlighted area is deleted. I don't realize it until I look up on the screen to check what I wrote. Why is this happening?

2. I try to select something and the mouse won't do anything. I can't select anything and I can't insert anything in the text. I press and press on the mouse and nothing. I leave it alone for a few minutes and it starts to work properly. Why?

3. I try to fill out a form and nothing shows up on the screen.

4. Suddenly, out of nowhere, the font on the screen is 5 times bigger than it was and I haven't touched anything.

Ok several questions need answered before we can assist you.

1. what o.s are you running

2. what software are you using to edit said file

3. what keyboard and mouse are you using

are you using the apple extended keyboard the one with the arrow keys and number pad

or the smaller one without the number pad

please let us know thank you

You need to tell us more details - which Mac, what OS X version, what keyboard and mouse, how much hard drive free space, etc.
Particularly - does your keyboard have a battery?

And now you've killed the forums
Just kidding. Odd things happen here sometimes.

@Ellenrk:

You didn't tell us what kind of a Mac you're using? If you're using a Mac notebook (MacBook Pro, etc) check the condition of your battery. If the battery inside is swelling it will put pressure on the trackpad and cause all the weird things that are happening to you. If you don't know how to open it up or the battery is non-removable, take your Mac to an Apple Store and have them check it out.