Hii..For Railway Group D exam..The main & Important Thing you have to do is General awareness.

there are lots of Sites available In which You can get free resource of whole General Awareness..

For another Subjects like Maths & Logic..You can focus on RRB mock test which you can get On Different Exam providing agency like

1-iexambee

2-Olieveboard

3-testfunda

4-gradeup.

Analyse your mocks very carefully & learn new things from it.