If you choose to try some guerrilla gardening, it is very important that you pick a safe but neglected area of public land. You do not want to get into trouble for trespassing on private land. Check your country's legal position on guerrilla gardening now. Attitudes vary across Europe. 

"The kind of guerrilla gardening that I advocate is mostly public land that is neglected. To me this is a fantastic resource for us all. This is our land. It's public already and if it isn't being looked after, if it's been forgotten about or abused, or attracting anti-social behaviour then imagine what it could be instead."

Richard Reynolds, TED Talk (watch here)

You will need to check the area for broken glass and dangerous waste first. Be aware of busy roads and your personal safety. 

Think small: Plant in all the small, grey corners of your city. Some poppies by the bus stop... lettuce on the street corner... sunflowers by the recycling bins. A lot of small actions will green the city! You do not have to take big risks.