There are probably several ways to do this. For a more comprehensive understanding of making new planes you could consult the help guide in Solidworks. But for speed here is a way I have created the tapered box you are looking for.
First create a basic centrepoint rect. on top plane
Then if you hold ctrl (whilst the top plane is visible) and drag from the corner of the plane you can create a new one parallel to the top plane. Drag this to the distance of the height of the box:
Then go ahead and create another centrepoint rectangle on this new plane, which corresponds to the upper bounds of the box. So without the planes shown it should look something like this:
Then to create a new plane at the required angle is easy. Go to your features > reference geometry > plane
Then create a plane referencing two parallel sides of the two sketches as shown below
Once created you can now sketch on this plane. I used convert entities for the upper and lower lines then joined them up and then finally extruded outwards the sheet thickness.
Follow this procedure for the adjacent side
Then mirror the bodies using the default front and right planes to finish off. And then you should have a tapered box touching at each edge which you can then apply a weld to.
This is probably one of a few ways to do it. You might also want to look into sheet metal design if you are doing a lot of sheet metal work but for now I will keep it basic. The advantage of doing it the way I have shown is it is now driven by the two original sketches so if I do need to change the bottom or top side lengths it is as simple as changing the sketch dimensions and all the bodies will update perfectly.
Let me know if you need anything clarified.
Hope this helps.
Wow, thank you very much! I didn't expect a tutorial with images. That's great! I'm sure I'll be able to get on just fine with that.
Also, with regards to using sheet metal, I have decided against using that for now as my first impressions from it, is that it Is more orientated towards the bending of sheet metal, which isn't used in what I'm doing for my first project. My impression is probably wrong, but for now, as you have said, I will keep it simple and will explore sheet metal once I get my bearings with the simple stuff.
Once again, thank you for taking the time to explain it to me.
Not a problem, glad to help. We were all beginners once