Hi there! Welcome to SF! I am also an 18-year old dude who is fairly new to this thing, let me tell you, you'll learn loads here! This is a great community. While I too am new, so my knowledge is woefully limited, I will try to help you to the best of my ability.
My first piece of advice is that denim goes with nearly everything. Unless you are trying to pair black balmoral oxfords or tassel loafers with jeans, it's difficult to run into problems. I'm not a big fan of jeans that are actually black/dark gray, but extremely dark indigo is always fantastic. I love pairing my browns and reds with jeans. In general, the deeper the indigo, the darker I like to go with my other colors.
If by trainers you mean running/athletic shoes, I would suggest not wearing them with your daily wear outfits. Much as I love my running shoes, they are for running, not for style.
As you are 18, I'm guessing you're on a pretty stringent budget (I sure am!) so I would start scouring Goodwill and other thrift stores. You can get some truly ridiculous deals on things there.
If you insist on buying new, I find that sperrys offer reasonable quality for the price, but take into account that these are glued and rubber soled, and will not last like a properly crafted pair of shoes will.
Good luck!
