I'm going to mention Paint anyway because that is your most immediate and free option. But other ones?
Paint.net is absolutely free, and I used it for pixel art for years, but it's not really meant for pixel art, and after learning a certain other program, it's difficult to go back. I know...
some people use
GIMP, but I couldn't tell you if that's good for pixel art. Textures, sure, but I couldn't figure out heads or tails of GIMP. I had a lot of difficulty trying to learn it, and I'm not really interested in doing so.
But the
best thing you can get for pixel art and probably textures too is
Aseprite. It's easy to use, works on the lowest end of systems, comes with lots of helpful features, continues getting support, and has plenty of folks in our community who know how to use it and can help you out. But either you better pay the cash or know how to compile it yourself (which is free). There's a trail on the site if you want to give it a shot before you buy/figure out how compiling works.