Recently I bought Canon 1200D with 18-55mm and 55-250mm lenses. I am interested in street photography.
Hate to say it, but I think you bought the wrong gear. But that's just me with my specific ideas of what street photography is or should be (prefer street shooters like David Salomons), where the photography is typically done with a wider lens than normal, and the context of the street around the subject is as much the focus of the image as the subject.
In this situation, you may want a camera that's a bit more discreet (i.e., less noticeable) than a dSLR combination. Most of these types of shooters prefer a mirrorless or large-sensor compact camera with a fixed lens, like the Fuji X100T, Fuji X70, Ricoh GR, Nikon Coolpix A, etc. etc. But this assumes you want wide angle street photography.
If this is your primary reason for buying an expensive camera and you intend to focus your efforts exclusively in this area, you may want to consider whether you should return the gear you just bought and get something better-suited to what you want to do.
That's not to say you can't do street shooting with a 1200D. And there are a lot of other things a 1200D does those cameras can't. Some folks define street photography as street portraiture which is done more with longer lenses from a distance. For this, the 1200D is probably a better choice with the 55-250 lens.
I would like to know which lens should I use, I don't want to buy new one right now (may be later).... I read lots of article about same and found 35mm lens is the best one.
35mm isn't the best--it's simply the most popular focal length/field of view that many street photographers prefer. But some like to go wider to 24mm, and Henri Cartier-Bresson, the father of the photojournalistic style that is street photography, used a 50mm most of the time.
Remember, too, that 35mm is the full-frame equivalent. On a crop-body camera like the 1200D, this would be closer to 24mm (24mm x 1.6 => 38.4mm). Just as a 50mm on full frame is closer to a 35 on crop, and a 24 on full frame is closer to 16mm on crop.
Possibly one lens to consider if you stick with the 1200D, is the EF-S 24mm f/2.8 STM. It's incredibly small and will help the camera be more discreet. But again, this will all depend on what it turns out works best for you. I'd recommend shooting with your 18-55, and then later analyzing the EXIF information of your shots to see if there's any specific focal length where you sit.
Which settings I should use?
No one can tell you this. This is absolutely an it depends type of situation and you need to learn your camera and settings on your own to discover what best suits in each situation. But as a starting point, Weegee used to say "f/8 and be there". :) F/8 is a small aperture setting and will require a lot of light, but it also gives you a very deep depth of field, so focusing is far less critical, and it will be using your kit lenses close to their sweet spot (where they're the sharpest). Given that street doesn't give you a lot of time to take the shot, you may not want to be fumbling around with achieving focus lock to get the shot.