I have Munkres' book on Topology. For higher Physics (beyond standard model, string theory, etc.) I know we need to have an understanding of differential geometry, etc. that assume knowledge in topology.
You don't necessarily need topology to learn basic differential geometry. But, yes, string theory uses fairly heavy topology. I'm a topologist, not a string theorist, but I know something about what kind of math is used there, even though I don't know much string theory.
My question is how much should I study from Munkres' book? I know that it is useful to study till Tychonoff's theorem (chapters 1 to 5), and some of the chapter on algebraic topology. Do I have to study anything else besides that?
I'm not sure you need all that stuff, even. Chapters 1 and 2 in Munkres are really the core material that you use over and over again in topology--connectedness and compactness. Another important concept is partitions of unity. Other things you can look up as needed. Part 2 is good material to know--the fundamental group and covering spaces. When you start reading a book like Munkres, you might want to try to learn it all just because it's there, but you can move a lot faster if you just concentrate on the most important bits that will be used later on. I think I might have used Tychonoff's theorem once or twice outside of topology, but I use connectedness and compactness on a daily basis. If you want to learn more analysis, you might use different parts of the book than if you were just wanting to go deep into geometry or topology.
String theory draws on more advanced algebraic topology than the basic Munkres Topology book--homology, cohomology, homotopy theory, characteristic classes, fiber bundles, K-theory, and index theory. For that, there are quite a few books. Hatcher's Algebraic Topology is good (most of the material aside from the appendices would be relevant). And Munkres also wrote a separate book about Algebraic topology. I wouldn't worry about the other stuff too much, just yet, but the next thing I would read after learning the contents of Hatcher would be the first chapter of his unfinished book about vector-bundles, available free on his website (as is the algebraic topology book itself).