I have downloaded a few fonts now, and I want to use them in a picture. How do I use it in apps like PicsArt, Pixlr, or Font Studio? Please answer soon, because I love all the fonts on this site!

You have to install the font in your OS.
Look the FAQ : http://www.dafont.com/faq.php

What does OS stand for?

You need to install the font in the same computer where those programs you mentioned you are using are installed.

The instructions on on how to install the fonts depending on your OS (operating system - like Mac, Windows, etc), you will find at the URL Menhir posted.

BTW if those programs are on phones, I do not know anything about that. I do not have a smartphone and I am still wondering why I couldn't get my dumbphone to connect to wifi.

Depending of your computer : Windows, MacOS, Linux, etc.

Have you try to follow the link I've give you ?

You post your message with Android. Are you trying to install a font on a phone ?