A bit more - basically adding to what's said here already.
I use Photoshop for making clothes textures and outfit shapes. Often I make an initial fabric pattern in Paint! and then enhance it in Photoshop.
You need to know how to make transparency using alpha channels to make Targa files to upload to
SL. Sounds complicated, but fairly simple once you've learned to follow strictly the set procedure. There's a lot of tutorial info on the official
SL forum - I think under 'textures'. Transparency is important if you want to make garments that have more intricate shapes than can be achieved just using the in-world sliders.
I use the CMFF templates creating when the clothing textures. They are free downloads, so if you damage one by saving wrongly, you can always get another! The templates have lots of layers with different giudes on - when you come to save your texture, you need to turn off (i.e. make invisible) all the guide layers, just leaving your actual texture (the Ps process I use requires one to add a blank transparent layer too, but there are various methods).
It's best to try out textures on the Beta Grid, because uploading is effectively free. It still costs L$10 per time, but you always have lots of L$ although they are not 'real' in that they can't be exchanged for RL currency like on main grid.