hi! youll find the city very large and so much to do you will either love it or hate it! heehee. but to your questions:
that guy with the girl, eldoth. he is a evil bard, and the girl is skie, a thief who will join your party too. (pair, like dynahair/minsc) so do or dont, as you will. she is tough to take out of the building without killing guards and taking severe reputation penalties anyway. you can take him in and dump him if you like.
entering the city first time around will be easy. you saved the mine remember? direction in there is somewhat hard to describe. talk to folk who have *names* and enter lots of buildings. the quests will find you, if you listen and read your journal, you wont get too lost. later in the game after the return to candlekeep, you have a choice - presented to you on the bridge. depends then, easy or hard.
some antidotes (purple jar) and oil of speed (patchy yellow) and healing potion (brown) are cursed like the scrolls. use the white jars of haste only. if the description says its murky or something, dont trust it. sell it.
in the gate you can buy stuff you need. plenty of stuff.
hope i helped some. see ya!