Welcome to the forum, I grow my bonsai in northern IL, and have a place just off Hwy 43, near South Haven Michigan, I've been to Lansing a few times. Have friends there.
In your climate, in Michigan, summer is an okay time to work on junipers. NOT a good time to work on maples. So for the maple wait until spring, when the buds just start to open, that will be your time to repot. You can pick up good bonsai media and a training pot between now and spring. Actually for the maple, it could really use a few years of growing, even if your final image for it is smaller than it is right now. I would move it to the size larger nursery can next spring, and let it grow another season or two. Maybe work it hard 2018 or 2019.
Junipers are best if when you work on them, you do it once, one day, then let it recover for 12 months or more. Compared to the California juniper growers, your growing season is only 1/3 the length they have, so don't let articles from California lead you to believe you can do 6 different major operations in a single growing season. You have time, anytime now until about middle or end of August you can do a repot. Or you can wire, or wire and prune, but don't repot, prune and wire. That would be too much.
To get a thick trunk, you need to let the juniper grow, even if your final image is for a much smaller tree. You could move it to a larger nursery can and leave it grow. Or, just to get practice, and see a bit what it is all about, you could pot this in a smaller bonsai pot. If I were going to repot, I would not remove any foliage to "balance" what you take off in the way of roots. Reason is the foliage hormones control how much the roots will grow, you want the tree roots to colonize the pot quickly after repotting, so the year you repot, don't remove any foliage. The roots will recover more quickly, and then the following summer you do your hard pruning and styling.
Or, leave it in its current pot, and do your pruning and styling. Having an undisturbed root system means the foliage will respond quickly to your pruning.
All options are available, what do you want to see these trees become?
HOw tall>
How thick the trunks?
any particular style?
Ideas?
Both trees are young enough they are blank canvasses.
it is mid june here in Lansing, Michigan and I just bought these two trees a Juniper and a Japanese Mapple from the nursery. What should I do first.: Transplant to a pot or trimming and wiring? Or it is not a good time to do anything because the time of the year.? 
