There's no real downside to taking it in high school, if you think you can do well on it. There are some potential downsides, but it won't hurt you in any way.
You take it at an approved testing site for clep. Generally, it's a university campus. I have no idea as to what the schedule is for taking it, because it depends on the individual testing location. It is computerized.
It...sort of is like an AP. It has no essay section, a multiple choice section that's longer, but easier than the AP multiple choice. The downside, however, is that it isn't really that impressive to colleges compared to AP tests. You might get college credit at smaller colleges, but it's credit that will hopefully be covered by a good AP score. A few flagship colleges (the University of Texas, for example) take certain clep tests, but those are generally in areas that aren't covered by AP tests. Top tier private schools like the Ivy League schools will almost certainly not give you credit, although it might factor into their decision making.
You can get college board to send the testing report to your university of choice, where it will be added to your file. You can put your score on your application, but there's no fixed box for it because clep testing is generally oriented towards college freshmen looking to get out of introductory classes.
In conclusion, you've got nothing to lose by taking it, save the money you paid to take the test, but it won't give you many, if any, benefits that aren't covered through the APUSH test, besides maybe showing dedication to the subject or something like that - no tangible benefit. Have you considered the SAT US History test, since you generally need a few of those to apply to top tier colleges?