Diet is extremely important when it comes to the growth and health of your hair. As I explain in my blog, "What to eat for healthy hair", a diet which includes sufficient sources of protein and iron is essential for hair to grow properly.
Omega-3 fatty acids and vitamin A are also important, not only for promoting hair growth, but for protecting the scalp from issues such as dryness and irritation.
There are many other practical tips which you can follow to promote hair growth, such as avoiding using heat on your hair as much as possible (through hair styling tools like straighteners, curlers etc.), conditioning hair regularly, and avoiding using hair dyes.
Although these steps can promote quicker hair growth, it is important to be patient! Growing your hair may seem to take forever, however, if you follow these tips, this should help speed up the process.