I've read that Bitcoin is not what it was used to be, or that mining is not profitable anymore.
You can't mine bitcoin with your computer anymore. Bitcoin is now mined with specialized hardware called ASICs. You can still mine some altcoins with your computer and sell them for bitcoin, but unless you have very cheap electricity, you will likely end up at a net loss once you factor in degradation of hardware costs as well.
Also I've read that to make money it is better to work with altcoins.
Depends what you mean by "work". Most altcoins only go up in price when bitcoin is going up in price, and dump harder than bitcoin does, so longer term you are likely to lose money on altcoins.
And what is the fastest way to earn BTC?
The fastest way to earn bitcoin is the same as the fastest way to earn any form of currency - work for it. If you have a service you can offer online, such as programming/coding, web or software development, writing, marketing, etc. then you might be able to sell your services for bitcoin. If you have a product you can offer for sale, then you can sell it for bitcoin. However, by far the quickest way for you to earn bitcoin is going to be to buy some with fiat. If you have a job, great. If not, getting a job and using your income to buy bitcoin will be much faster and earn you much more than almost any other option as a newbie.