I live in Germany. While I was working during the World Cup match between Germany and Brazil last night and couldn't watch the game, I was still able to keep up with what was happening, thanks to the honking and cheering from the sonic condos outside. While that particular street noise was fun and helpful, other outside noises can be terribly annoying. But there are some belongings you can do about it.
Use your exterior walls
Nothing absorbs noise like a thick wall of books. Consider moving your bookshelves to align the exterior walls so the books help absorb some of the sonic condos. Fabric is another good insulator, so a solution that moves your closets, clothes racks, or linen storage to outside walls can help, too.
Get thick curtains
Heavy curtains can also help muffle the sound. The thicker and heavier the curtains, the more effective they will be. You can even buy special noise-absorbing curtains designed to assist wipe out annoying sounds.
While I'd love to cover my entire apartment in velvet one day, for now, I'm using IKEA's Werna blackout curtains. They're very thick and heavy, they don't cost a fortune, and as a bonus, they also block out all the light that would otherwise leak through. If the noise drives you crazy, you may want to consider hanging two sets of curtains.
White noise
I find that a fan or white noise machine helps a lot.
Reinforce the windows
A lot of noise leaks out of the windows. While you may not be able to convince your landlord to replace the windows in your standard rental apartment with better ones that prevent noise, there are a few DIY options you can consider. Covering the windows completely would probably turn your apartment into a little dark box, but cutting some sheets of acoustic foam to fit the windows will give you an option that you can put in at night and then disassemble when you want the windows returned to you.
Earplugs
I like to save the soft foam earplugs that planes give out on international flights. They are comfortable enough to fall asleep and avoid almost any noise.
Trust the time to fix it
In the end, time is likely to take care of most of the problems. I grew up in a major city, so sirens and car horns don't even register as noise to me. When my husband first moved to a city, he couldn't sleep for weeks due to all the noise. He was convinced that he would never sleep again. sonic condos before long he got used to it and now he can sleep with anything. He even slept during the World Cup game last night, and that was so bad I thought the windows were going to fall off. Give it a little time and you may find that you don't need any noise cancellation solutions.