Start by opening GIMP first. Then,
from the Main Tool Bar and Go to > Windows > Dockable Dialogs > Click on "Fonts"
This will open the Font window where you can scroll down and select the font you want.
By defaults its the upper right area. You can click on attachments to see the full size.
Attachment:
Font-01.jpg [ 302.36 KiB | Viewed 1869 times ]
Now Press the Letter "D" and then "T" on the keyboard.
1st or next change the Font Size to the PX you want for now.
2nd or Next Left click with your mouse some place in your work area for your text to display.
Now type some text you want.
3rd scroll through your font list on the right and click on the font name you want to use.
Attachment:
Font-02.jpg [ 262.76 KiB | Viewed 1869 times ]
Now if you need to make adjustments, like size and color, for now
make them in the Text Window on the left and not the one above your text until you master this.
There are 2 different ways to do all this, but this is the sure way and the simplest way for
learning Gimp.