I want to publish my original paper in medical journala. But i dont have enough money to pay. Is there any journal offering free access or publication?
Nearly all journals will cover the cost of publication if you can demonstrate that you do not have the funds to pay publication costs. I would contact the journal and explain your situation and see what they can do for you.
I would just go ahead and submit it to an open access journal (perhaps one of the BMC family) and if/when it gets accepted, mention that you can't afford to pay. I would hope that the journal would cover the cost on your behalf. Maybe you will be indebted to them to do some peer reviews in the future.
I forgot to mention that although most high-impact journals have page charges and submission fees, some of the 'lesser' journals can actually be free or just a have a minimal submission fee. You have to simply go ahead and read what each journal has to say on the matter and make up your mind regarding what works best for you.
In response to Ryan's suggestion, I also believe these BMC journals would publish if you can demonstrate hardship, but I would be upfront about it. As much as I support open access journals, their model of publication works by charging the authors the cost of publication and thus can at times be more expensive than regular print journals. BMC journals can be costly, but I've seen lower publication costs from hindawi (around $200), but the trade-off is that they are relatively new and unknown.
I would identify the best journal for you study and simply contact the editorial staff and see what they can do for you.