Plain form: When you're talking to friends, family, colleagues and classmates, subordinates, kids, animals and in any situation when the level of conversation is clearly casual, like making small talk in a bar or something. Students may also use the plain form when talking to their teachers (since in Japanese society teachers are seen to be in loco parentis), but they always use the honorific title "Sensei" to avoid overfamiliarity.
Many older people, especially women of the "old battle-axe" variety, routinely use the plain form when addressing everybody, regardless of the situation.
"Ore" is used by males (females may use it, but only if they wish to project a particularly butch image and accept the consequences). For male-speakers "boku" is safer - more relaxed than "watashi", less overtly macho than "ore"- in general or casual conversation, especially at first. For females "atashi" is more or less similar to "boku".
Actually, you're more likely to sound offensive if you use an over-familiar 2nd-person pronoun to an adult you don't know well (or at all), such as "kimi" or "omae" or "anta". For that reason it's usually a good idea, especially at first, to avoid such pronouns and let the context of the sentence do the work for you. English-speakers are often bemused by this, but it's an easy enough practice once you get the hang of it. And don't forget, in Japanese conversations you can always use your interlocutor's name in lieu of the pronoun. It may seem strained and artificial to westerners, but among the Japanese it's a very common practice.
One of the biggest traps westerners fall into when speaking Japanese is the over-use of personal pronouns. For the Japanese using watashi/atashi/boku/ore etc all the time when it's obvious+ from the context that you're talking about yourself comes across as a little crude and childish. In any case, you're obviously aware that Japanese personal pronouns can be a bit of a minefield, so it's usually a good idea to use them sparingly until you develop the knack of using them appropriately. And that knack comes with experience and the inevitable+ faux pas that you have to make while you get the hang of it all).
Incidentally, if you're face-face with your interlocutor, and you want to say something while making it particularly clear you're talking about yourself, you can just point to your nose. That's Japanese body language, a non-verbal indicator meaning "(your)self".
And finally, you're A_Dark_Embrace, using a different handle, right? There are certain telltale characteristics in your writing style and the contents of your OPs that definitely point to that. Also, the way you only appear in your own OPs, and never respond to others' OPs is quite A_Dark_Embracesque.
