i want to start developing ios apps on my lap (windos)
i dont have mac
so how can i start

You have a few options. The best, short of an actual mac, is to run a virtual mac OS X on your PC. I haven't done this myself and some people have noted that it tends to be a performance killer. You'd need a high end machine to get good results.
There are a few frameworks out there now that compile to iOS such as Mono, Xamarin (for visual studio), phonegap. I haven't used these either as I went and bought a cheap, second hand mac laptop when I needed to do iOS programming but they seem to get a lot of usage so they must be good for something.
If you eventually think of buying a cheap laptop make sure you get one with good hardware. Apple are releasing new iOS versions and SDKs pretty frequently so an older Mac machine can become redundant pretty quickly.

I experimented with Yosemite in a VM on a fairly ordinary Core 2 machine (but with 8 Gig RAM - essential) amd performance wasn't noticably different from a 4Gig Mac Mini with a similar processor. With an SSD it was faster than a Mac Mini. (You should get advice on the legality of runnning OSX on a non-Apple computer.)