Is it on your toe nail, or your toe? I'm assuming your nail. If so, it will take a long time to sort itself out. You can get over the counter stuff (scholl do one, curanail etc) which are specifically designed for toe nail infections. As the new nail grows through it should, if the treatment is working, be normal and healthy. You can so get these nail lacquers on prescription. If this doesn't work, your doctor will be able to prescribe tablets to help clear things up. Either way, it will take 9-12 months, so you'll need to be patient!
As a last resort, they can remove the toe nail.
Being diabetic increases the risk of developing fungal nail infections, as does spending a lot of time in trainers etc. They can spread to other toes, so make sure you get it treated.
I had one about a year any a half ago. Dropped a champagne bottle on my toe (don't know what was more upsetting - the pain or the fact that the champagne was wasted!). The nail went black and then turned quite brittle and yellow in colour. I had to use a nail treatment for 18 months. It's a bit like a nail varnish. You put it on your nail once a week. Mine cleared just after 10 months but my doctor advised me to keep using for 18 months to stop the infections recurring.
Hope this helps!
Em