This is obtained from the finishing the very first mission. There's no time or score constraints to it.
If you've played AC Chronicles: China or Russia, the basic moveset is more or less the same so it should be a case of brushing up on your skills.
For the uninitated, the game will guide you through most of the level and offer up hints and tips for you.
My major advice would be to keep an eye out for splashes of red colour. These usually indicate you can climb on it. (red ledges, red ladders, red paint on walls you can climb etc.)