Congratulations! It is such a wonderful and fascinating hobby. As it is for your son I would HIGHLY recommend that you start a fishless cycle (there is a wealth of info on this) so you can eliminate the risk of heartbreak and have a great positive start to keeping fish. Everyone has to start at the beginning so if things don't always go quite right don't be too hard on yourself, we have all made mistakes and learned from our actions. Read up as much as you can and if unsure of anything or if you have concerns never be afraid to ask on forums etc for help no matter how trivial it may be, most people will be more than happy to help you.
There are a whole wealth of beautiful species to keep. Just make sure whatever you decide to buy will not outgrow your tank, in shops you will see many amazing little fish but staff might not always inform you of the monsters that they can quickly become.
Any fish that you see/like you will be able to find min/max sizes/tank sizes/conditions through the internet. Always research your tank friends instead of buying spur of the moment.
Also be prepared to wait until your tank is fully cycled and ready for you fish, this part of the process can test your patience but is so crucial in keeping the best interests and health of your fish in the long term. Also (not sure of the age of your son) it would be a nice idea to share,learn with him and show/teach him how the different stages of getting your water cycled and maintainence of keeping your eco system.
Enjoy and remember help is always there for you!
p.s I would highly advise that you buy a decent test kit, these things last a long time and are a vital part of keeping fish and seeing what is going on in your tank that isn't visible to the eye. They are widely available and I would personally recommend API freshwater master test kit. You can get cheap strip tests but in my opinion are highly unaccurate.
the type of fish you get depends on the size of your tank as well as other factors like do you have a heater?
