It depends what stage you are at with your reasoned reflection and what arguments you have been exposed to.
For an analogy: imagine that you have gone walking in a freezing forest and you have gotten lost. And no one has found you for months, but somehow you've managed to survive, against the odds.
I am justified in believing you are dead, as is everyone else apart from.....you. Our evidence that you are dead is that you have been missing for months in a freezing forest. You too know that you have been missing for months in a freezing forest, but this doesn't provide
you with good evidence that you are dead; for you have apparently cast iron evidence that you are still alive. Of course, if you subsequently encounter angels and such like in the forest, then it would start to become reasonable for you to believe that you had died as well. So, it all depends on what apparent evidence you have and its probative force.
Other things being equal - that is, you have no basic belief in God - then the default is not agnosticism, but atheism. To be agnostic at the outset is, well, silly. Just to assume there is some evidence God exists apropos nothing is not remotely reasonable. And the fact this world does not at all appear to be the kind of place God would create and place innocent people in, provides you with powerful reason to disbelieve in God.
So, you should start an atheist if you are reasonable. The burden of proof is squarely on the theist. And to think that the brute possibility God could exist provides you with some reason to doubt atheism is, well, unreasonable (anything is possible - but it is not reasonable to believe anything). At the outset you have no reason to think God exists, and apparently good reason to think God does not exist.
That should continue to be your position until or unless you encounter arguments for God's existence. If those arguments are valid and have premises that seem self-evident to rational reflection, then you should - if you are reasonable - start to take seriously that your belief that no God exists is open to reasonable doubt.
There are such arguments, of varying probative force. There is no question they exist, for they are discussed by philosophers to this day. But you should satisfy yourself of this by exposing yourself to them and seeing what your own reason says about them.
It is at that point that it would become reasonable for you to adopt an agnostic position. For now you have apparent evidence that God exists. Indeed, if you continued to be confident in your atheism you would have discovered that you are unreasonably committed to atheism. After all, being reasonable is not about what you believe, so much as the manner in which you believe it. And so if you continue to be an atheist despite being unable decisively to refute arguments for God's existence, then you have discovered that you are unreasonable.
Reasonable agnosticism, then, requires appreciating some of the force of the arguments for God's existence. Someone who thinks the arguments for God's existence are rubbish, but nevertheless calls themselves an agnostic is a bit of a twit. (Needless to say, this place is full of them). A reasonable agnostic thinks there's a good case for God.
If just one of those arguments appears to be valid and have premises that are far beyond any reasonable doubt, and you have sincerely attempted to refute it and failed, then it would be silly to continue to be an agnostic. For now you have the best evidence that God exists. Likewise, if the cumulative power of all the arguments for God raises the probability that God exists significantly over 50%, then you should stop being an agnostic and describe yourself as a theist. (There's more than one way a thesis can be proved - sometimes it is by one zinger of an argument, sometimes it is by a cumulation of weaker arguments). A reasonable person's beliefs are responsive to evidence: responsive, that is, to reasoned arguments.
Needless to say, above I am describing how I came to believe in God. The important point, however, is that agnosticism is not the default. You have to earn the right to be an agnostic - earn it, that is, by appreciating some of the probative force of the case for God. And it is an inherently unstable position. For God does not both exist and not exist. So either atheism is true, or theism is true. And yet the reasonable agnostic thinks the apparent case for both positions is roughly equally matched. They must at the same time, however, acknowledge that one set of those appearances is illusory.