It depends upon the context. If the comment being made is regarding something someone is saying, for example, giving an opinion on the speakers content; you might say something like the following:
"He's babbling" - much like a baby makes noises that have no meaning
"He's full of it" or "He's talking to hear himself speak" - meaning the speaker is not only ignorant, but also arrogant or wanting to be the center of attention. A similar term is "He doesn't know what he is saying."
"He's trippin'(tripping)" is a more modern slang that has multiple meanings and so is often qualified with an additional statement. It can pertain to either a person's speech or actions e.g "Man he's tripping. He don't know nothing about how to fix no computer problem." Has ties to things people might say or do when on an LSD or other hallucinogenic drug "trip"
For actions, the following may be appropriate:
Similar to "out of depth", it might be said that someone is "in over their head", meaning beyond their experience range.
For someone who has obviously overestimated his capabilities it can be said that they "bit off more than they can chew" similar to trying to fit something into your mouth that is far too large for it.
Another saying that also has multiple meanings or can be applied to multiple contexts is 'Wrote(writing) a check that his butt can't cash." As in committing to doing something, or attempting to do something for which they lack the capabilities. That could be either resources, or in the context that I think you are looking for, knowledge and/or experience.
Less derogatory terms (or they can be) for actions, usually applied more to professions are:
Layman - someone who is not an expert in a given field.
Tinkerer - usually applied more towards physical applications as in someone who doesn't know much about computers or devices, but likes to take them apart to try to figure them out.
A Hack - this one has many different nuances but can be a professional or non-professional who has limited education or experience in a subject matter. It can also mean someone who does a poor job on purpose, but it can definitely be used to describe someone who wrote an article about a subject while knowing too little about it. e.g. "Why would an uneducated hack like Huckleseed answer a language question?"