To solve task you need to write program, copy sample data you were given along with problem statement to its input - and then copy back the answer which you program gives.
Answer is checked, and if it is correct, you get points.
Simply click Tasks at the top menu. You will see the list of tasks here. Choose any task and click its name to move on its page.
It is recommended to start with Beginner's Problems.
As stated above, to solve the task, you will need to perform some calculations on input data given below problem statement and enter your answer into dedicated field.
Since usually such calculations will require to write and execute a kind of programm (in any computer language you want), you will be asked to enter program source also. After your answer and source code are both entered, you can submit them.
You have several options to run your solution for producing answer. Please, read Running your code page.
Note also, that tasks are arranged into volumes and you can switch between them (clicking dedicated link at the title above task list). We are going to arrange tasks by difficulty and by applications (i.e. beginners tasks, physics tasks etc.), though it would not necessarily always be so.