How to approach the "customer" (your employer)
If they asked you to do the job and you honestly told them what were your capabilities, you have nothing to worry about. Throughout the following months try to keep them informed on your progress and immediatly tell them if you have some kind of problem that will slow you down.
You just have to be sure that everyone has the same expectations, and you don't deliver an underperforming product to an overestimating customer.
Keep your resolve and be confident in yourself. Nobody was able to make a fully working and well written software right from the start.
Learning will probably be slow at the beginning, and you WILL find obsacles. Don't lose hope and slowly grind your way around them. When you will be done, you'll think everything was a lot easier than you initially thought.
Learning a language
If you never touched any programming language or GUI tool, choose the one you think is better suited for the job (based on languages supported by the libraries of the instrument you're developing for) and try to put up some simple example program, a small CRUD application with a backend of some sort will show you the most common problems and pitfalls for a beginner. For example, you could try developing a small app to manage your friends addresses and contacts.
If you have no tight time constraint, don't be afraid to try, fail, and try again. Learning from your own mistakes is extremely important for a developer.
Keeping your code clean
There is lots of documentation on the internet about this topic, and learning how to divide and reuse your code will surely make your life easier.
As a rule of thumb, if a method/function doesn't fit in your screen or a class has more than 7/8 methods you should probably ask yourself if the method/class isn't doing too much, and if you should move part of your code somewhere else.
Having well organized code will reduce the chance of bugs in your final product, and will make it easier to fix them or add new functionalities if they ask you.
Asking lots of questions and reading lots of examples
Websites like Stackoverflow are born exactly to help programmers share their knowledge and get better at their jobs. Most basic questions already have an answer there, expecially for more popular languages. If you don't know something, don't be afraid to ask.