Is there a way to lock the font size on my computer, it changes too easily.?

If it is just so you can see the text to read it, press on the  CTRL key and while you hold that down tap on the PLUS sign near the backspace key.  Tap on it a few times while holding CTRL down continuously.     

.  If you want to decrease it hit CTRL and the minus sign beside the 0 key to shrink it.  You can do this for images or text.  It is your control for your monitor.

On a guess, do you mean Microsoft word?. If so, do the following

Open word

In the top left window (Font) click on the small arrow in the lower right hand corner

Now, make the necessary changes, then click "Set as default"

how does it change? it's not something that can be changed easily.

unless you are in the habit of mashing the keyboard keys.

where is the font changing at? you can PM me if you need to.