It's difficult to offer a complete solution to your problem. Unwanted noise can be difficult to deal with, which is why in a professional scenario most people would opt for a sound-proofed studio when they can.
Having said that there are some things that might help improve your sound. For one, make sure your Blue Yeti is set to its cardioid polar pattern. This will mean that to some extent the mic will reject sounds coming from behind it. Also you should consider your distance from the mic. If you are too far away from it you'll naturally need to have the volume set higher, which will make the outside noises louder. I did a quick skim read of the Blue Microphones website and they recommend you be roughly 6 inches from the Yeti, and certainly no more than 1 foot away. In your recording software it might also help to use an EQ to try and reduce the background noise, but this will be a delicate balance between reducing unwanted noise and affecting the vocal quality.
This will probably not be a perfect fix, but if you haven't already implemented these things, they will probably help out :)