I am currently aiming to go for a postgraduate in mathematics, but currently I am in a bachelor degree in IT. Although IT is fun and all I have recently developed a liking of maths primarily the idea of functions, sets, graphs and relations from my introduction to discrete maths and now I want to know more from calculus and linear algebra for a more solid conceptual level.
Furthermore back in highschool I am very slow at arithmetic so I never get a good grade on my exams for either not having enough time, and I wanted to improve both my concept and quite bit on my computation. May I ask where can I get started (in order if possible) to learn maths properly?
Depending on what you are looking for, I would suggest different resources. I will assume that you have a basic knowledge of functions, sets and relations, and thus do not need a basic reference on those.
A good way to start is by reading some classic books: "Algebra" by Grove, "Topology" by Munkres and "Calculus" by Spivak provide the very basics that you should know, and more. Probably all of them are available at your local library. Even if they are only three, they will take you some time to read, even more to master, and do not even cover what a freshman should know in his first year. If you feel ambitious, consider: "Linear Algebra done right" by Axler, "Algebra I, II, III" by Cohn (I believe the modern editions have different names, but the content from the old editions is priceless), "Calculus I, II" by Apostol, "Complex Analysis" by Ahlfors, "A fisrst course in Algebraic Topology" by Kosniowski and "Ordinary Differential Equations" by Tenenbaum and Pollard. This is by no means a comprehensive list, but it will get you started: most of the above listed are books that you can use as an initiate and carry through most of your learning.
For further reading, with some time of getting familiar with books in the area, you will recognize most of the publishers that provide great advanced mathematical content, some of them being Springer-Verlag (with their characteristic yellow and white books), Cambridge University Press, Wiley and Princeton University Press: just look at the references that the books above give.
Nowadays there are multiple sources of reliable Mathematics online (including Stack Exchange), and a lot of Professors post their notes free of charge on their websites: Terence Tao has Linear Algebra notes, Sergei Treil posted his exceptional parody of one of the books above mentioned and Keith Conrad has expository papers that cover a wide array of subjects, not to mention the multiple Universities that provide free content through their websites, like the MIT, I also suggest you skim through them.
However, in Mathematics the most important thing is to do your own work, to get involved and to try and solve problems. You may find that you understand a concept or even that you feel that you know everything about a particular one. Do not be fooled, if you have not done any exercises or problems (you don't have to look for them, almost every single Mathematics book will leave proofs to the reader and have exercises at the end of the chapter), your knowledge is not cemented and the base you are building will be weak and crumble when more complex concepts appear. Make sure that for every hour that you spend reading, you put another hour into problem solving.
Disclaimer: The above lectures are unfortunately biased towards a more algebraic approach of Mathematics. If that is not what you desire, I can do my best to provide further literature complementing the above.