So finally, after many months battling with Blender because I couldn't export (because I'm dumb and didn't check if I had extra meshes) and my own procrastination, I made my first object. It's only a box but I struggled so hard doing it. Anyway, my issue here is that, as you can see, I don't know if the mesh is invisible or what because I can only see the inside of my object. I don't know if it is a problem with TSRW but in blender the texture seemed right when I applied it at the mesh. Sorry if it's there a solution or a thread with the same problem as mine, I don't speak english so I didn't know how to search for it aside from invisible mesh.

Your normals are inside out, you need to flip them. In Blender select the whole thing and from the Shading/UVs menu click "Flip Normals" and export it again. If it's still busted in tsrw, there's probably some more wonkiness going on with the mesh. Go back to blender, select the whole thing and from the vertex menu > "remove doubles", then with everything still selected from Shading/UVs > Recalculate normals. After that you can put the texture on it to check if it's still inside out or not (if it is - flip normals and this time it should stay fixed).

Thank you so much! You were so helpful.

No problem, good to see it worked!