TITLE: "Advice to young mathematicians"
QUESTION [185 upvotes]: I have been suggested to read the Advice to a Young Mathematician section  of the Princeton Companion to Mathematics, the short paper Ten Lessons I wish I had been Taught by Gian-Carlo Rota, and the Career Advice section of Terence Tao's blog, and I am amazed by the intelligence of the pieces of advice given in these pages. 
Now, I ask to the many accomplished mathematicians who are active on this website if they would mind adding some of their own contributions to these already rich set of advice to novice mathematicians. 
I realize that this question may be seen as extremely opinion-based. However, I  hope that it will be well-received (and well-answered) because, as Timothy Gowers put it,

"The most important thing that a young mathematician needs to learn is
  of course mathematics. However, it can also be very valuable to learn
  from the experiences of other mathematicians. The ﬁve contributors to
  this article were asked to draw on their experiences of mathematical
  life and research, and to offer advice that they might have liked to
  receive when they were just setting out on their careers."

REPLY [2 votes]: Here is my answer in short

Use YouTube videos .Some of them are good and explain basic content.

The Math Sorcerer  channel is an excellent for math advice and
inspiration

George Polya’s Mathematics and Plausible Reasoning vol 1 &2  These are great books to build mathematical skills

Circle around a particular problem to try solving it,instead of a
attacking it head on.

If you can’t solve a problem, go to another one and when     you go to bed, think about it and forget it. Let “Deep mind”,
ie,subconscious mind ,find a way to solve it. You might be surprised it finds hints for your conscious mind to solve it

Use this site,it is wealth of info.