When landing in Chicago, you won't have much choice when exiting the aircraft. Just follow the signs to Exit / Passport Control / Baggage claim, and you'll end up at immigration (Passport Control). There shouldn't be anywhere else you can go.
Since you are on an immigrant visa, you'll most certainly have to see an officer in person (and not just use the kiosks). I don't know if there may be any additional paperwork / procedures which may lengthen the operation compared to regular visitors or citizens/PRs. Be attentive to signs to enter the right queue.
After passport control, you'll end up at baggage claim (again, nowhere else you can go). Check the belt number for you flight on the screens, and get your luggage.
Note that if you spend some time at immigration, your bags may have been taken off the belt and be standing next to it to make space, or if you were substantially delayed (your flight won't be shown on the screens in that case), even moved to a corner somewhere. In that case, look for a "baggage claims" counter, they should be able to direct you.
Once you have (all) your bags, you go through customs. In most cases this is just a matter of giving the form to the officer, though of course they may ask more questions and/or inspect your bags.
From there, you'll end up in the "public" area (what people in the industry call "landside"), on the arrivals level (lower level in Chicago T5). Here the "only one option" path ends.
Normally, you would use the people mover ("ATS") to get from T5 to T3, but it's currently closed for upgrades, so you'll have to use a shuttle bus.
The O'Hare page on the topic is clear as mud, but it seems the shuttle bus departs from the lower level at T5. You'll then get to T3 on the upper level, where you'll need to find the check-in counter for your flight, and check-in/drop your luggage before the deadline (45 minutes before departure time for AA).
From there, you'll go through security, and to your gate, to board your next flight.
Edit on time and plan B
Based on the information you provided, my guess is that you are arriving at 1:45pm and leaving at 5:10pm, with a check-in deadline at 4:25pm, which means 2 hours 40 minutes from door opening to check-in done if your incoming flight is on time.
It should be doable if there are no major issues, but it doesn't leave you much margin, especially considering the terminal change, ATS being closed, separate tickets (so having to check-in again) and new immigrant processing.
You should probably have a plan B ready just in case. Either make sure your ORD-BOS flight can be exchanged or refunded up to the last minute (but don't forget to do the change in time), or see if there's a cheap later flight you could book (though last minute bookings are usually much more expensive).
It seems that at this time the next flight to Boston is actually the following morning, so you may have to spend the night in Chicago if ever you miss you original flight. Good thing is you'll have all your luggage so it shouldn't be a problem. However some hotels won't (or can't) allow a person under 18 to stay alone there.
Note also that AA does not seem to fly the route every day at this time.