There are many online preparatory courses available for competitive examinations like SSC, Railway, Banking, etc. Some of these courses don’t cost anything. There are also various channels on youtube that you can follow.
Since your subject is Mathematics, you will have some advantages over the other aspirants in these examinations. If you start preparing from your undergraduate, you will get enough time to prepare for these examinations.
If you are looking for a government job through general competition then it is the right time to start your preparation. As you are pursuing UG with Mathematics, you will have the advantage over other aspirants in the aptitude and data interpretation section which you can solve easily.
For preparation, you can do it either online or offline. You can even prepare for these exams even free of cost with the help of YouTube. If you are financially stable, you can look for coaching classes.
Various YouTube channels provide updates with the material, guidance, virtual class, etc which will be very helpful to you.