:D it is not an easy thing to make your projects look realistic. You have to have or developed an experienced eye for detail, materials, shaders, lighting, models, camera angles (compositions). You have to know what is your final MOOD.
Sometimes MOOD can be different depending on the client and what is your client goal. If it is your personal project then you can go crazy :)
It takes time and a lot of work. Experimenting with everything. Doing a lot of projects until you will be happy with your result. Look at a lot of real photos of architecture online and so on. Google is your friend.
Try to take a photo from google of an exterior shot and try to re-create it as close as possible. That way you will start developing the understanding what you need to do to make it look more realistic.
Keep in mind that this is a very very short explanation what do you need to do :) It is a lot more to it. It is a different kind of ART.
Go here for inspiration :
http://www.mir.no/work/Good Luck.