[hnglt]

https://en.wikibooks.org/wiki/Formal_Logic

This wasn't my introduction to logic, but I like to skim it for refreshers, and it seems well-written enough to me.

My introduction was What is Mathematical Logic? by Crossley et al., which is a master class in terseness and probably more suitable for someone who's already got a foundation in logic. I must've read the first three chapters three times each, and the rest fewer times than I should've :-).

[rasulkireev]

Yeah, I love wikipedia, this is probably a good start :)

[hnglt]

Sorry if this isn't what you're looking for. I just read your other reply that you're looking for philosophical logic. I've got no experience with that at all and can't give a suggestion. But good luck with your learning!