Jo, just wondering, have you tried out the missions yet ??? Best advice is to complete all the side tasks before you even attempt doing missions for the mob in Portland. Do the taxi, firefighter, vigilante, and MEDIC !!! *hehe* missions first. Granted, it's a little hard to get to those spots in time, so don't be discouraged. Some missions later in the game are VERY frustrating, and quite a lot of patience (and game saving) is required.
Keep on trying Jo. Best to complete those side tasks (like what I mentioned up above) while you're still in Portland, coz Portland is relatively small (a LOT smaller when compared to Staunton Island and Shoreside Vale, which will be unlocked later in the game), and easy to drive around. After a while, you'll get used to all the roads and even know the shortcuts. Practise as much as you can and you'll get the hang of it !!!
And yeah, it definitely IS possible. You just need to know the roads like the back of your hand (which requires you to drive around OFTEN). Driving skills gotta be top notch too, coz you need to go fast without HITTING too many things, or else your taxi will blow up or the passenger will get out of the cab. Practise...that's the only way to get 50 fares, let alone 100 fares.
By the way, did you manage to do the hooker trick ??? I've been trying and trying but they just don't seem to want to get into my car.
Here's a good site for GTA FQAs, cheats and hints:
http://www.gamefaqs.com/console/ps2/game/31127.html
Good luck with the game, Jo !!! I'm just about to install it into my computer as well...quite sick of playing it on the PS2, so I'll let you know my progress. =)