Hei @veek91 , sounds as a really good oportunity!
I'm going to talk out of my knowledge, but If I was in your position this is what I will do:
-Learn and master some software like Substance Painter, 3D Coat, or (I didn´t know about it before) but in this case, Maxon Painter.
-Look what the style the company does and make some portfolio pieces with the same style (Handpainted, PBR or whatever, I'm a beginner at this)
If you don't want to modeling and whatever, you can do things like this:
-Pick some free models and make them awesome. Also, you can do practice things like tileable textures and this kind of things, and also make your own models (simple ones, don't spend a lot of time on modeling couse you need to focus on texturing if is what you want) and texture them with different versions for example.
-I think Shaders and materials are very important for texture artists, watch some tutorials about it, here i can't say more, I really can't recommend anything to you becouse I don't know, but I thing that Allegorithmic also have a lot of free tutorials and stuff.
Hope this can help you at least a little bit.
Well, just one more thing, don't wait!! Send them something (that looks profesional), If not you probably lose the oportunity. Later you can always send them your portfolio with the things you've learned.