Will taking diet pills to loose weight affect my chance of conception?

Depends. In general the best way and the most effective way to lose weight and keep it off is through exercise and healthy eating. Diet pills generally do not provide longlasting weight loss. If you are overweight, then losing weight will improve your chances of conception if done through healthy, sustainable means.
Not necessarily. With obesity, the estrogen hormone is not balanced leading to infertility. Many women become very fertile and unexpectedly become pregnant after a bariatric surgical procedure due to the weight loss. The pills themselves shouldn't affect fertility but may be contraindicated during pregnancy.