Firstly, I'm talking about a still pretty young juniper tree, I don't know the kind, but it has small thin needle leaves if that helps

Ok, so last Christmas, I got a bonsai from my aunt and uncle. I actually was avoiding getting one because I was fairly certain I'd kill it but my mom let it slip that I like them so here we are! It's still alive but I feel like it's not doing to well. I water it every other day and give it indoor plant food once a week. Also the man who sold it said to replant in a year, but I also read that you should replant in the spring so I'm thinking of waiting till then, but that still leaves me with the question to how to replant it. I was originally thinking of just using the same soil I use for my other plants (regular potting soil with coffee grounds mixed in) but then I saw that they need a layer of substrate too, so, would it be alright to use my potting soil as long as I add the bottom layer of substrate? Or should I get specialized soil?

Second question: I want to start some bonsai from seeds, I'm planning to use Japanese maple seeds and evergreen pine seeds. The question I have is whether or not I can use the soil straight from the ground around here. Pine trees are our most common tree around here and Japanese maples seem to do well too (I live in a forest) so logically I was thinking that this soil would be good for them, but then again, bonsais are different than just growing a regular tree so I'm torn. Most of the soil around here is red dirt, some of it is clay like (enough to where I could actually make clay from some of the dirt). Stains everything to hell and back but the trees around here seem to like it.

Anyway! I'd love to hear y'alls thoughts on this and I'd love any help you can give me!
(side note, I do know to stratify the seeds before planting!)

It is not doing well because it is inside!! There is no such thing as bonsai seeds, they are tree seeds. Bonsai are not grown they are created. No it is not advisable to use garden soil. It is not sterile and will contain all sorts of pathogens that are potentially harmful. Look on this web site for how to grow various trees and style into bonsai.

There is a section of the web site marked How To which answers pretty much everything. There is specific information about juniper and a good discussion of soil, potting, etc. Give that a look.

..but first of all: place it outside , inside it will die, no doubt about it.
Do not water on a schedule but when soil feels dry
Ps: there is no such thing as an indoor tree....