Double negative does not always mean positive. When people say things like "I didn't see nobody" or "I ain't got nothing," they are emphasizing the negativity. These sentences aren't grammatically correct, but some people still say them.

By double negative, I take it you mean sentences like: I ain't got nothing (I haven't got anything) I'm not doing nothing. (I'm not doing anything) I never did nothing (I didn't do anything.) In informal speech you may hear such constructions, although they are not really correct. What they mean is in the brackets, and as you can see a sentence like 'I ain't got nothing' does NOT mean 'I have something.'

Not always. Colloquially, double negatives are used a lot, especially in phrases involving 'ain't'. In fact, the double negative with a positive meaning is virtually obligatory when using 'ain't. A person who uses 'ain't' would be very unlikely to say 'I ain't been anywhere' : they would always say 'I ain't been nowhere'. This usage is "correct" according to the grammar which governs this non-standard usage. Needless to say, it is not advisable for a non-native to use these forms, unless (a) their English is of a very high level (b) they are among native speakers who are also speaking in this manner. There are also other cases in standard English where you might use a double negative without it becoming a positive. For example: 'So you said that you don't like that shirt?' 'I didn't say that I don't like it. I said that it doesn't suit me. It's a nice shirt, but it's just not my style.'

Double negative is always positive. If you want to emphasize the negative you can use adverbs. You don't have to go to the party. You don't ever have to go to the party.