HI, what is a project plan? how to write it?  

First step, outline what you want to do. What exactually do you want to accomplish. Decide on what your main goal is. How you want your project to pan out. Once you've taken notes look it over. What do you already know? What other things do you need to learn? If you've never written a computer program or software, look into taking classes. Your local college may have some you can take, or you can get an online degree. Plan out what you'll need to learn what corses you may need to take if any.

Once you've completed that, research into some of the finner details. Make sure yours isn't going to be considered a rip-off of someone else's work. It can take quite some time.

Write pratice codes, and test them out each step of the way. Take your time. ask around for more help if you need.

I think that you need to make a survey first before you write such a project plan. I mean, you should make clear of your project and make some investigation for all the information you need for your plan. I think that you can surf on the Internet to find similar information for the writing of such a plan.

A project plan is an overview of what you want to accomplish. It will not only describe what the computer program should do but every phase of the project.
This should include, but not limited to, the following.
Describe the results of the analysis from a business perspective.
What business function will this improve.
Describe the computer hardware needed to perform this task.
Mainframe application or PC. Will reports be required and what they will be.
Define all of the input and outputs of the program.
Provide a high level description of what functions and or calculations are needed.

You say you want to write this for an education software project.
Does that mean that it must be very general so as to cover all, or most aspects of eduction or is this for one specific type of education?

The final product should be understandable to both the people who want to use it and to the development team that will put it together. You do not need to know a programming language to do this. Your specifications however need to detail out all the functions required. The programmer can take it from there.
The more familiar you are with what they need however will make it better for them.
I have developed programs from very high level specs. They will come ask you questions to work out details.

Something I found very helpful in both developing specs and developing from them was this statement...."Always design with the end in mind". If you first look at what the final result should be you can always work back from there.