You should condition your hair everyday and do conditioning treatment once or twice a week after straightening. I don't suppose there are any harmless straightening technology since it will grow out anyway. If your hair is thick then you can use some hair products that conditions and smooths your ends. Never ask for so-called expert advice from the sales, look through the product info yourself. Use a reputable name, some cheap ones from Mannings and Watson's can provide the same effect.
I think you should be grateful for what you have, some people are dying to have waves and you wanna straighten them? Have you considered using some hair ornaments to decorate your hair instead of wasting money over straightening it? There is a lot you can do if your hair passes your shoulders.