Anika, it's going to be okay.
Okay, pants. It's hard to find pants that fit. In my size (which is fat size

) They think all big ladies have the giant, huge, puffy hips, thighs, and buts. Which I don't. Gods forbid that maybe, just maybe, humans all come in different shapes regardless of "dress size." So, big lady pants all have these extra rounded out areas in the hips, which on me look like ears.
My advice on pants is to look for anything that is "boot cut" or "straight leg." Even if they are a bit big, maybe you can find something that hangs nicely. My son has your issue. He's only six but he already has this issue where he's too tall for the pants that fit him around the middle, and too skinny for the pants that fit him in the legs. Luckily little boy pants come with elastic in them that you can make them smaller on the waist, but he could easily put a pillow on his butt and it would fill in the puffiness....
Shoes... not trainers. Not heals. You're going to need a comfy flat, but not a dress shoe. Something dark and closed toed. Maybe a boot or something like that.
As for the work place. If they don't train you that's their own fault. You won't fail because they don't train you. They will fail you by not training you. If your boyfriend works there the people probably are not that bad, unless all he does is complain about them. It will be hard at first to get to know people, but you're a nice and kind person. I'll give you "mommy" advice. Just be yourself and you'll make a friend.

Not everyone will be friends, of course.
When I first started I was really shy here. Now I have some "work friends" who are pretty good. And I made one friend who is actually a good friend. So, just think positive, it will be okay.
As for bosses... hopefully they are not mean. Like I said, unless your boyfriend has cmoplained, then I wouldn't worry right now.
You're going to to great! I just know it! Good luck looking for pants. I really hate shopping.