The advantage of Paying yourself first and saving or investing is that you are able to build up a nest egg for your pension or shelter yourself with an emergency fund that can weather unexpected circumstances.
How much should you save/invest?
This is very personal and I'm not a financial advisor who can consult you on this. There are however some guidelines I can give you which should help you determine the answer to this question.
First of all, you need to know how much you spend and on what. If you have no idea about your spending habits, I strongly suggest you start journaling every little cent you spend for a period of 3 months. You could do this in an Excel sheet or use one of the many apps that are available for your mobile phone.
Once you have done that, you'll know what you spend your money on and you can start digging into which expenses are inevitable, and which you can avoid or reduce (do you really need a subscription to Netflix , Amazon Prime, Hulu, Spotify and all that other stuff?). Remember, that every sacrifice you make now, will result in a much bigger return in the future, you are investing in yourself.
Depending on your situation, you will come up with a (fixed) amount, or a percentage of your income that you can do without, as if you have never received it. As a general guideline, I would suggest to start with 10% of your paycheck, and re-evaluate on a yearly basis, but this is all up to you. This money will be the building bricks for your future wealth and once you get started, you will see that it gets addictive and you'll want to save more and build that reserve faster.
What should I save/invest for?
We need to distinguish 2 goals here, an emergency fund to cover unforeseen situations on one hand, and a nest-egg, for your (early) retirement.
The first thing to do is to build that emergency fund, a strategic reserve as it were. General guidelines say that it should amount to what you need to survive for a minimum of 3 months, but I would strongly suggest to gather enough for 6 months or even 1 year of expenses. You don't need as much as 3/6/12 monthly incomes, when push comes to shove you can survive on the bare minimum and as long as you have shelter, clothing and food and a minimum of comfort you can get through that period. Your budgeting exercise should help you to determine how much is needed here. You'll want to keep these funds at hand, save or invest it in something that can easily and quickly be converted back to cash.
When you have that in place, you can build for your future, really start building your wealth, so you'll start routing your savings into other vehicles that have higher returns and can compound over time.
What should I invest in?
Again, I'm not a financial advisor, so you'll have to do some research yourself here. Savings accounts barely generate any returns these days and the same goes for bonds, so you'll need to look out for other, more rewarding investments like index funds, commodities ( gold , silver ), retirment savings plans with tax benefits and maybe even dedicate a little to more risky assets like cryptocurrencies ( BTC , ETH).
Keep these things in mind!