If you sit outside, you must find a place where you can sit unobserved and undisturbed by people or by stray animals. Take a few measures to ensure that the time you sit will be high-quality time. Disconnect the telephone and doorbell. Pick a time when things are quiet. In a city, this is difficult.
Good times are at dawn, before the city wakes up, and dusk.
Dinner time is good, in particular, since most people will be eating quietly. Try to ensure that your ten minutes of zazen will be fully ten minutes of sitting only. The main thing is to sit every day. If you are fastidious about this, even for ten minutes, you will progress without a doubt and perhaps you will decide to sit for longer periods. Daily practice is the key.