In my campaign the main antagonists are a country run by chromatic dragons (mostly greens) and in a couple of weeks the party will be free of it's current enemy and I was thinking of throwing a young dragon at them (probably green but maybe black) the party will be 5th level at that time so I think they are finally strong enough to handle it. The question is how would you go about creating a dragon adventure?
[MENTION=6801286]Imaculata[/MENTION] gave you great advice/questions right out of the gate.
In my experience, dragons do best when foreshadowed well. You never should, IMHO, "just throw a dragon" at the PCs like you might a handful of bugbears. Well, arguably depending on your style of game, you could make that mandate true for ALL monsters, but I think for dragons it's always true regardless of your genre/style.
Anyhow, something to consider is its age. A young dragon is probably freshly kicked out of its parent's lair (in which case, what are its parents up to?), still searching for a lair (and quite vulnerable, potentially allying with evil humanoids to protect itself), or has just found a lair of its own (possibly after displacing the previous residents who are angry/fearful/scheming).
Also how would you include kobolds as antagonists for a 5th level party?
This I can answer with confidence. Use lots of them. Fight as nasty and dirty as possible. Assemble a list of simple traps given an extra level of kobold meanness.
Also, check out the kobold 5e resources in my sig. Lots of options for diversifying kobold stat blocks and making them work better with each other and their trap-laden environment.
You might also see my Dragon Mountain conversion (even though that's for 4e, there are tons of cool ideas and tricks that you can translate - that entire boxed set was basically an exploration of "Tucker's Kobolds").