i got this from m friend

Liberty City is a tough place for the uninitiated. Missions can end in hilarious failure, which will definitely get you and your buddies laughing, but it’s still failure. Take these tips with you to help make your xperience more enjoyable and explosive!
Taxis
A taxi cab can be your best friend. You can use the Left Bumper (L1 on the PS3) to hail a cab in the street, and while already-occupied cabs won’t stop for you, you can still catch up with them at a light, hold the Y (Triangle) button longer, and pull the current passengers out of it and get in yourself. The nice part is that this doesn’t count as a crime, either.
Use taxis as a quick way to get around town. Spend the extra money to skip the trip, and you can ensure that when you want to do missions, most of your time is spent actually doing them rather than driving around between points. A nice side effect is that there are fewer opportunities for Little Jacob and such to call you and demand to hang out with you, so you don’t have to get off-task or turn people down nearly as much.
If you befriend Roman by hanging out with him enough, you will be able to call for a private car at any time. This is a nice perk since many missions and situations will leave you sitting out in the middle of nowhere with no decent cars around. Oh, and one more thing - Little Jacob eventually starts giving you jobs to ferry around drugs with time limits to boot. Not only do you not have to use the hoopties that LJ leaves the drugs in, but you can get in a cab, set the destination, and skip the trip to finish these missions very quickly and easily. It may not be the most fun way of doing it, but it works.
Money and dying
The best way to “lose” in GTAIV is by death rather than by getting arrested. You see, you lose a percentage of your money going to the hospital, but lose all of your weapons if you go to jail. Just pay attention to the warning signs - if you’re on foot and a cop gets close, Niko will put his hands up and is about to get busted. Mash the A (X on PS3) button to run away, and even if you immediately die, it’s still better than getting busted. Unfortunately, there’s no way to avoid an arrest if a cop opens the door of your stationary car and points his gun at you there, so that’s a good time to load your game - or just avoid getting arrested that way altogether.
The biggest issue with having a big stockpile of cash is that you lose more dollars for going to the hospital. For this reason, I recommend you stay stocked up on ammo and weapons so that deaths don’t cost you as much overall, and believe me, you’ll start to use all that gear if you’ve got it. Just keep in mind that there is an Achievement on the 360 for getting together half a million dollars, so if you’re going for gamerpoints you might want to play really conservatively with your cash. Personally, though, I was able to beat make the half million mark after finishing the game’s final mission - and that was with another half million already spent on trips to the hospital.
Learn to shoot
I know this one sounds pretty obvious, but the firefights in GTAIV expect you to use cover almost constantly and will put the hurt on you if you’re exposed for too long. Using the lock-on is a good idea in my opinion, so what you do is get behind cover with right bumper (R1 on PS3), hold the left trigger (L2) all the way down, then find someone to kill. Click the right thumbstick once to use any zoom your weapon may have - even pistols get a little zoom - and use the right stick to find the spot that your enemy has exposed or will expose once he emerges from cover to shoot at you. Just wait for it, and as soon as it’s open, pop off a few shots with your gun. Headshots make a huge difference here, too, so get those if you can. Also, flick the right stick to switch targets, or just let go of the left trigger, aim in the general vicinity of the guy you do want to target, and pull it again.
Grenades
Grenades are valuable tools that may cost a lot at the gun shop, but are well worth the price. Any mission where you’re sent to kill someone that’s in a car and doesn’t know you’re coming can be easily ended with a quick grenade toss. There aren’t many, but if you are able to drive up on someone without alerting them, don’t get out of your car and don’t pull out a submachine gun. Just drop a grenade in front of their car at a stop light and drive off. Easy mission win!
But they’re useful in firefights, too. Niko can toss a grenade pretty accurately without even looking, and most enemies that are focused on shooting you won’t run when they know it’s coming. This is even more useful when there are explosive things around, especially cars. Use them if you’re low on health or can get several at once, but they’re not worth using just for one enemy.
Cars as cover
Of course, enemies themselves don’t have grenades or rockets to send at you, so using a car as cover is great when there isn’t any around. You can use these in impromptu shootouts with the cops, or even make good use of them in quite a few missions where you’re chasing someone and they finally get to their location and get out. Of course, if you plan on using the car again, you might want to find another one nearby to soak up all the damage, as it’s really frustrating when you’re trying to make your getaway and your car catches on fire with the last bullet fired as you speed off. That often leads to jumping out of the car at high speed, and I can speak from experience when I say that that will often lead to you getting run over by an oncoming police cruiser. It’s funny as hell, but then you realize you’re dead.