Use a heavy bag to practice keeping a proper fist with powerful hits.
When I first got mine and would throw hard, heavy combinations, on the 4th or 5th connection I used to have a terrible habit of letting my thumb start to poke out (just like a thumbs up symbol). It's a subconscious thing, really, it just happens, but it can lead to a snapped thumb or worse with a poorly placed punch. The only way to train yourself out of a bad habit like that is to train, to practice. There's no other way around it, really.
To specifically answer the question, though, when I make a fist my thumb ends up lying across the knuckles closest to my fingernails on my index and ring fingers and ends just about midway between the 2nd and 3rd knuckle on my ring finger.
[EDIT: If you need to clench a lighter or something similar in order to make a solid fist, you're not curling your fingers tightly enough. There should not be room to work with like that in a clenched fist. I mean, yeah, you can throw a punch like that, but you're definitely opening yourself up to some pretty stupid hand injuries (and that's speaking from tons of experience with shattered knuckles, hairline and metacarpal fractures.) I don't really fight but that's one of the ways I blow off steam nearly every day, and before I got some legitimate guidance I had at least one fucked up hand almost 24/7.]