If the infection has been long-lasting, you would be better off going to your GP. He/she should take a sample of the nail, send it off to the lab and , if necessary prescribe you a course of tablets (possibly cream as well). They are sometimes hesitant to do this as the treatment is quite expensive and you have to take the pills for 3 to 6 months.
If you are trying self-treatment first, your pharmacist will advise you as to the cheapest, most effective cream, but the secret is to apply it religiously twice a day for several weeks. It can take ages to get rid of fungal infections.
Good Luck.