You can sign a contract without any problems. But assuming that you are not a legal adult, you or your guardian can void the contract. Details depend on the country, usually until you are eighteen or shortly after.
So let's say I want to buy your website for $50,000, and we write a contract that is absolutely fair, nobody trying to take advantage of your youth and inexperience. When I sign, that's binding for me. I can't refuse to pay the $50,000 saying you were to young.
You or your guardian however (most likely your parents) can change your mind and make that contract void. Which is a big risk for me, because If I work really hard to make that website a success, and when you are close to 18 years old it is worth $200,000 then you can void the contract, and get the website back that is now worth four times as much. You can see that I'm not quite willing to buy your website under those circumstances. (And I can't put anything into the contract to avoid this, because you can void the contract with all the clauses that I added to protect myself).
So if you find someone willing to pay, you can sign a valid contract even at 16 (or even younger), but because of your young age it may be harder to find a buyer.
On the other hand, if the contract was that you do some work for me and I pay you, I'm bound by the contract (I have to pay you once the work is done), but once you've done the job, it doesn't matter anymore if you void the contract because the work is done.
And you or your guardian don't have the right to change the contract. You can fulfil the contract, or you can void it (which means the contract just stops existing). You can't change it.