Thanks for choosing Gibbon, and getting in touch via the forums. To answer your question:
Look at Admin > User Admin in the main menu, and then Import Users in the module menu. Note that the import is currently really a sync, but in v11.0.00 (to be relased January 20th 2016), there will be an import mode too. For details on importing, please check out https://gibbonedu.org/support/administrators/importing-users-into-gibbon/. To add users one by one, go to Admin > User Admin in the main menu, and then Manage Users in the module menu.
For fee management, please take a look at Other > Finance in the main menu. Here you can add fee details and billing schedules, and then start creating, editing and issuing invoices.
matsyuf, for tracking payment, please check the Finance module under Other in the main menu. At the moment there is no way to check the amount collected or expected to be collected, but you could use the Query Builder module to compose an SQL query to calculate this. Perhaps if you do not know SQL, you could consider hiring an outside consultant to do this for you.
1- Add student e.g Matovu Yusuf This can be done by going to Admin > User Admin in the main menu, and then Manage Users in the module menu. Once in Manage Users, click on the Add icon to add a new user. This user can be of any type (e.g. student, staff, parent, other), just set the role accordingly.
2- Add student to a class e.g Senior One If you want to make enrol a student, to make them active in a particular class, this can be done by going to Admin > User Admin in the main menu, and then Student Enrolment in the module menu. Click on the Add icon to enrol the student. You will need to first create some form/roll groups, which can be done under Admin > School Admin in the main menu, and then Manage Roll Groups in the module menu.
3- Indicate that the student has paid full amount Go to Other > Finance in the main menu, and look through the various options in this module. Set up any fees and billing schedules, you need. Then go to Invoicees, which will automatically set up your students to make them billable. Then head to Manage Invoices, and generate and issue invoices to students. When payment comes in, you can then edit the invoice, and mark payment (either in part or in full).
4- Then how i ca check the students who have paid the respective category This is shown by the status field in the Manage Invoices action in the module menu in the Finance module. You can filter by type, and export to Excel for further analysis.