Greetings! very excited to be entering this magical world of the bonsai...I recently purchased a small plum tree and a small cascading juniper...can I prune and wire them now (I'm in Massachusetts) and then repot in the spring or should I wait to do both in the spring? Susan

Welcome.
first lesson: bonsai is a marathon, not a sprint!! Sit back, look at your tree, turn it, look a bit more, repeat.
Best thing is to keep it alive for a year first, which is strangely enough already very difficult for a lot of people. Use that time to study horticulture, look at bonsai trees, take a ( online) lesson ( good ones here on the site), dig through youtube, go to your local bonsai club, ....
Everything starts with the health of the tree. For instance: where did you place your trees?

Thank you for responding so quickly! yes, I've been sitting frequently with each of my trees, getting to know them, studying their structures, etc...My plum is in a window that gets southern sunlit...I had my juniper outside but then it got below 32 so I put it in the garage...there are windows on the garage doors so it gets some light, but not a lot so I am a bit more worried about it getting enough sun during the winter....As far as classes, I've already taken the 2 beginner courses offered here and going to take the next one soon...there is a local bonsai store where I purchased these 2 plants but they are not offering any instore or online classes right now due to covid...if things open up in the spring, I'll enroll for some classes there....

32? that's hot !!! No, i know you are in of the the very few countries in the entire world that uses F .....
Your juniper can easily take -10 ( C of course), so no need to put it inside yet. Only thing you need to worry about is the rootball freezing over entirely (below -5 or -10 day and night during a few days in a row).
Plum as in sweet plum-sageretia? Indoor trees ( which is a contradiction) are always problematic, 99% of the help my tree is dying /wilting/... problems here are from people with indoor trees.
Well done on the classes!!
Could you post some pictures so we know what we are talking about?

Ok! I just put my juniper back outside...it has gotten in the teens (F) but I will keep it outside unless it gets below zero....my plum is natal plum (carissa macrocarpa)...I've attached pictures...thank you so much!!!

Both young plants, years too soon for a bonsai pot. Repot in bigger containers ( or in the garden for the juniper), and just let them grow bigger. Plum needs to be outside if temps rise again in spring ( if it’s still alive by then)

Would I repot them now or wait until spring? and I'm a little annoyed because when I bought them, they sold me pots and said I could repot in the spring...how big or how old does a plant need to be before you can start the artistic end of the process such as pruning and wiring? or would I prune these as they grow?

It all depends on what you want, how ( big or small) you want your tree to be. Of course you can repot in spring ( which is the time to repot, so not now) but they will not become bonsai just because they are planted in bonsai pots. There are no rules about how old a plant must be before one can start the artistic process but I think it’s not difficult to understand that you can never create the image of an old tree from a young pencil thick sapling. The smaller/ younger/ thinner the start tree the more difficult , the more experience and knowledge it takes to create a credible bonsai.

this makes a lot of sense...thank you for taking the time to respond to my questions...I think at least for now, I'll nurture and befriend them over the remaining fall, winter and early spring months and try to simply keep them alive!!!

A lot of the questions are answered in pretty good detail in the How-To section of the forum. It helps a lot taking care of the trees if you know some very basics about tree physiology, and there is some explanation of that in How-To.

You may have figured out, the Forum is international, and they love to make gentle jabs at those of us who use Fahrenheit, inches, etc. Every now and then you can jab back, gently.