First let me say that I am not an expert at wildlife photography; however, it has been my passion for many years. Taking pictures of wildlife is very different from taking pictures of flowers or your family. You are probably not going to be able to get very close to the wildlife without scaring it off, so you are going to need a zoom or telephoto lens. I carry a Canon 70 - 300mm zoom lens. This allows me to capture the wildlife's habitat and then zoom in and get a close-up shot of the animal as well. A lens with a longer capability would be better, but they are very expensive. They are out of my range at this time. I also carry a light weight tripod with me for extra stablization and better focus.
Depending on the type of wildlife you are photographing, you may need some type of cover, such as a blind. If you don't have a blind, be sure that the clothes you wear blend in with the habitat you will be in. Gray colors work well.
Always take an extra camera battery and flash card, if you are using digital. You will need to wear some comfortable shoes and I carry a small towel with me, as I will always need to wipe something down.
If I am going to be gone for most of the day, I will also take a bottle of water and an energy bar. I always carry my cell phone. Leave it turned off, but you never know what may happen and you may need to call someone for assistance.
I have written a hub on this subject, if you would like to read it. I don't think I am allowed to include the link here, but it is titled "5 Important Tips for a Successful Wildlife Photography Trip". I hope my answer here has given you some useful information. :)