Aseprite is a good program. Its relatively cheap and easy to animate in. Rotoscoping is a fun way to start, you have a guide for what to outline and can experiment with coloring, resizing and animation to build on then you can make small gradual changes. There is no substitute for time put in drawing, that's the only way to really get your own style. Also, if you are really trying to do this get a usb tablet, cheap is okay, entry level wacom tablets are about $30-50.

Actually I wouldn't recommend even bothering with a tablet that doesn't have a built-in screen. There are obviously a lot of artists who can create great art with a screenless tablet, but it takes a lot of training (depending on your personal aptitude, YMMV) to get good at it, and in the meantime pixel art is one of the styles where you can get by with just using a mouse. It's mostly high-res digital painting where you absolutely mus have a tablet.

Those Miniboss tutorials posted by @riidom look great, but they are for experienced artists.

What most pixel art tutorials (including those Miniboss ones) don't tell you, is that to be good at pixel art you first need to be a good artist, period. You need to learn the fundamentals.

For example, those Miniboss tutorials mention the term "anticipation". This is one of the classic "12 principles of animation" as told by classic Disney artists. This is stuff you need to learn outside of the tutorial, you won't go far at all if you rely on that tutorial alone.
Or if you don't know how to keyframe and breakdown a sequence of animation (the theory behind it).
Or if you don't know color theory (additive & subtractive systems, color harmony and contrast, color vision, light and material science etc.).

You should be able to design, draw and paint already. If you don't have artistic skill your work will look bad all the same, no matter if you're painting in 4K HD or pixel art.
If you have no interest in getting better at art, but still want to have beautiful graphics for your game, the smart choice would be to find an artist to work with you.