If you're using either Windows, Mac, or Linux, you can use FireAlpaca, it's freeware (ad related to the program at startup) and fairly simple to use. It's the first art program I've used and i still use it to this day since it works well enough for my needs. It works well enough for Homestuck stuff speaking from experience here so i recommend it highly if you don't have much experience with art programs, the only real issue is the "animation" feature basically just hijacks the layering system instead of featuring a standard animation timeline. Which causes performance issues if you're animating with a lot of layers of art as you have to duplicate that for each frame of animation. (It's weird, you have to put everything needed for a single frame in a folder, and if you just want to move one thing for the next frame, you have to duplicate that whole folder to make a new frame.) which isn't great but it's good enough to export to a gif then to put into ezgif and manually change the timings from there.

https://firealpaca.com/download/

If you're on Android or IOS... I salute you soldier for the battles ahead, every program there sucks. Medibang does have a free android and IOS version though (Medibang is basically just FireAlpaca with less features and cloud saving if you pay for the subscription for it. It's weird that there's basically two very similar programs being sold by this company, both of them with very similar free versions too.) But you have to watch ads if you want to do super complicated things like saving to a folder Android allows you to access outside of the app. If you're on Android specifically i found LayerPaint Zero to work, if a bit clunky, but i had to download an APK for that since it's no longer on Google Play and is no longer supported, but it doesn't have ads and it supports the same exact file format as FireAlpaca and Medibang (mdp) so you can transfer your art between devices while keeping your layers. (Without having to deal with PSD's, PSD is "supported" by all the programs i've mentioned, but PSD is not an open format so bugs tend to happen if you use more complicated features as Adobe doesn't release the specifications for the format, so devs have to basically make educated guesses on how it works.)