You can use tools such as Helium 10 and add your competitors ASIN's to see which keywords, organic and paid (PPC), they are ranking for and add them to your own listing provided they're relevant in your title, product description, product bullet points and written text via A+ content if you are brand registered. Amazon works like a Google search engine.
Add the most relevant ones to your product title, ensuring you are as close to maxing out the character length as possible. I would also, if possible, try and add relevant keywords to the packaging or product you sell in your hero image. While this does not impact SEO, it does impact Click through rate, be careful here, Amazon likes clear hero images not full of clutter or text.
Ensure you've maxed out your "generic keyword" sections, you get 5 to go through when you edit your product listing.
Also, your products will begin to rank better organically when you begin to rank higher via PPC, it's a twofold benefit. I notice on my own listings the more PPC sales I get, the more organic sales I get, I get about 66% PPC sales and 33% organic.
These are all general tips but a great starting place as I don't know what you sell. YouTube is a great resource too including the likes of "My Amazon Guy"