The P0455 code basically means that there is a very large leak or no purge flow from your EVAP system (to learn more about this system please read my write up about code p0440 and p0441). When the computer sees that the fuel fumes from the EVAP system are not flowing into the engine when they should be it sets this trouble code.
There are not too many things that can cause this code because there are not too many things that can cause this large of a leak.
If you have this code the first thing you should do is check your fuel fill cap and the fuel fill pipe. You might even want to just replace the fuel cap as it is very likely that it is what is causing this problem. If neither of those things fix it, then you should open your hood and look for missing or damaged hoses.
If you still cannot find the problem then you should take it to a shop that has a "smoke machine". They can introduce smoke into the EVAP system and see exactly where it is leaking from.
If you have already tried the above steps and still can't find the leak, then it is unlikely that you will find it. If that is the case, asking a professional mechanic or taking your car to a trusted repair shop will be the best way to go.
If you still have any unresolved vehicle problems or questions, you can ask an auto mechanic online. For expert answers specific to your vehicle's make and model, I recommend JustAnswer Car. They have a large pool of certified mechanics to answer your questions for a small fee and you can also browse their answers to other users for free.