Use the gear you already have and start with the 18-55mm. For most street photography, a wider focal length is often easier because it lets you include more of the scene and work closer to the subject; on your 1200D, try the wide end first, around 18–22mm, and see what feels natural. The “best” focal length is personal: some people prefer being close with a wide view, while others prefer standing farther back for tighter shots.

A practical starting point is to use the 18-55mm stopped down to about f/5.6 or f/8. That gives you more depth of field, which helps when you need to react quickly. If changing settings in full Manual slows you down, don’t force it—street photography often rewards speed more than constant adjustment.

The main advice from the community is simple: go out and shoot. Afterward, review your images and note which focal lengths you used most and which look you prefer. That will tell you far more than articles about a “best” street lens.