arboricola has provided an excellent resource for you make sure to explore the rest of the site starting with the section entitled "Bonsai Basics" and later move on to the rest.
Congratulations on your new tree. Something you wrote raised a red flag for me though.
My 7yr old son has just given me a Bonsai and it says it is a Zelkova. It is quite beautiful with tiny little leaves.
Zelkova generally do not have what I would describe as tiny leaves. There is another species that is sometimes confused with Zelkova and this is Chinese Elm. This side by side comparison that I made may help.
The US 25 cent piece pictured is 15/16 of an inch or 24MM. The shoot on the left is from a Zelkova while the one on the right is a Chinese Elm. Aside from the obvious size difference note the difference in the growth pattern between the two as well, the Zelkova has a much more pronounced zig-zag effect.
So the first order of business will be to positively identify your tree. A picture would help but as they do tend to look similar it is the size/scale that will tell the tale. This is important because the two species, while related, can/should be handled very differently.
The Zelkova is most definitely an outdoor tree while a Chinese Elm CAN
be kept as in indoor tree. I say can because not everyone, myself included, treat them this way. If it truly is a Zelkova then you are in somewhat of a pickle, it simply will not do well indoors. If it is a Chinese Elm you are in a much better situation.
I live in a flat with no place to put it outside but now am concerned as I hear that these tiny trees need to be outside.
Yes, bonsai is primarily an outside activity, though some species can be managed as indoor trees. Anything that I have that will not survive my winters, ficus and Crassulas, are brought inside in the fall and returned outside when the weather warms. Everything else stays outside all year although they do get some form of winter protection, but that is another topic.
can I repot it now or should I wait until spring.
No, do not re-pot now, if for no other reason than you, presumably, do not yet understand bonsai soils. They bear little resemblance to what you may think of as soil, in fact "soil" is somewhat of a misnomer Please read the sticky thread about [url=https://www.helpfulgardener.com/forum/viewtopic.php?t=3422]bonsai soils.[/url]
what position can I put it in to ensure its growth and survival
From your description it does not seem that you have much of a choice. The window is OK for now but be aware that what you perceive as bright light is probably too dim for your tree. If you are serious about this I suggest that you look into some form of [url=https://extension.missouri.edu/explore/agguides/hort/g06515.htm]supplemental lighting.[/url]
Something as simple as one of the new Compact Fluorescents will help but there are better, more expensive, choices. I don't recommend conventional incandescent bulbs, the wavelength is all wrong for general growing and they produce too much heat in relation to their output which means that they must be kept farther away from your tree thus even further diminishing their effectiveness.
Use the search feature of the forum to search for the term "humidity tray", they have been described numerous time here in the past. This will help to counteract the dry conditions that is likely present in your apartment.