What to expect as a first job
Expect to be doing a lot of bug fixes and very little coding at first if you join a large(r) company; smaller one-man / startup type companies, expect to work your ass-off and to be disagreed with a lot, but be open to learn from seniors, they've got the battle scars to back what they are saying and if they are wrong, make sure to address their point with compelling arguments, if not, anything you say in the future will be shot down.
Which mistake we should avoid
How you handle critique from your mistakes is what makes you a great / average developer, mistakes will always be made. I was working on a project as a newbie with a large team (which later became a multi-billion dollar organisation) and the architect reviewed each line committed and if he did not agree with your commit, he would come running at your desk and shout at you if your commit was not of great quality - so everyone was nervous hitting the enter key after doing svn commit. After a few weeks of getting verbally abused multiple times, I stood up, look him in the eyes, and asked, "how do I fix it", he shouted, go read the docs, I replied, "which docs?", he shouted, "section _ of __, google it". Sometimes I would get it right, sometimes I went back and asked him to explain it to me and in the process I was learning rapidly directly from the architect and soon people started asking me stuff they were too afraid to ask the architect.
So man up and take critique if you're learning from it, if the critique is unfair, find another job, otherwise document what you've done so you don't make the same mistakes again.
What to learn to be a great developer
Everything, start with history, it'll teach you why certain things are done like they are, do as much theory as practical. Spend an hour each day trying to learn new things outside your regular work-day - 7 days a week and 365 days a year - you'll be amazed how quickly you advance simply by putting in an hour every day - over 10 years, that's 3000+ hours you've invested in yourself that other people might not have done and it will show - I average on 3+ hours a day depending on how urgent projects are that I'm working on. Do something using AWS, then try and do it with Google Cloud, build something in Node, then switch and try building another component using Spring and Java, make them communicate, try building UIs without frameworks, then try doing things using different frameworks, try figure out how to build complex data models using NoSQL and then build it using SQL and get a feel for the tradeoffs, etc
How to stay relevant in the work industry
Be great at what you do, have passion, then it doesn't matter what tools you know, you'll often get the job without even knowing the language simply because you show commitment to teach yourself what is needed to get the job done and have already demonstrated that you know a very broad spectrum of other solutions.