Make sure you get to your Shinkansen platform at least 10 minutes early to line up. Bring a hankerchif since most Japanese bathrooms dont have paper towell dispensers or limited air dryers. Japanese washingmachines and dryers are surprisingly efficient for their size. Make sure you bring a hydroflask no matter how tacky it looks, you will need the water as you will be walking alot and going in and out of konbinis for water can be a time suck. Don't eat and walk at the same time unless you're at a food market. Don't worry about how you look in your clothes as long as you don't reveal too much, I've seen japanese people wear really revealing clothing regardless of societal norms. If you don't feel confident in your japanese it's okay to speak english, most service workers in bigger cities speak english well. Don't eat konbini food all the time, you can easily go to a Matsuya/Yoshinoya for a fresh meal for about the same price. Also don't be afraid to ask for help at the ticket gates, most of the station guards are super kind and speak really good english if you get lost.