Level selection and level unlock scenes are extremely well-covered territory on Youtube tutorials for Unity. Start there. If the first tutorial you complete doesn’t show you what you need, move onto the next one.
Two steps to tutorials and / or example code:
do them perfectly, to the letter (zero typos, including punctuation and capitalization)
stop and understand each step to understand what is going on.
If you go past anything that you don’t understand, then you’re just mimicking what you saw without actually learning, essentially wasting your own time. It’s only two steps. Don’t skip either.
If that approach doesn’t solve it for you then nothing written in this little box will do it either.
And as Spiney points out, which levels are unlocked is simply a savegame system, nothing more, nothing less.
Load/Save steps:
An excellent discussion of loading/saving in Unity3D by Xarbrough:
And another excellent set of notes and considerations by karliss_coldwild: