GTA Online is an immense game. IGN's Community Tips page serves as a central hub for players to share their expertise, tips, tricks, and accumulated wisdom with other community members.
Read on and draw from the reservoir of Community experience!

GTA Online Day One Prime Directive: Steal a Good Car!

The first car you steal and fit a Tracker to is the car you're stuck with. No joke, you'll gain ownership of the first Vehicle you commandeer, and after that you'll have to purchase new cars if you want to keep them. So for heaven's sake choose a good Vehicle!
It's worth an extra few minutes of wandering to grab the ride you really want. Note that not all cars will be available for theft in the early game, so choose the best you can get.

Cheats Don't Work

Don't bother trying GTA 5 cheats in GTA Online. They don't work.

Connection Issues Blues Clues

Having Connection Issues on Day One? Don't despair! Players are experiencing black screens, server errors, and mid-game crashes. A particular sticking point appears to be the loading screen for the first online race. Players attempt to queue into the race and find the program frozen.

Save Up For an SMG

Ignore the temptation to spruce up your ugly day one duds. Your cash would be better saved for a shiny new SMG. Though you can't buy one until your Rank climbs high enough, squirreling away your early-game cash for an SMG will leave you much better equipped for success than a sport coat or new pair of shoes. Buy the cheapest clothes you can when the game forces you to visit the Clothing Store.
Link your Rockstar Social Club account and you can pick up a free Sawed-Off Shotgun when you enter any Ammu-Nation store.

Watch Out for Parked Cars

Experienced players with more money than they need can purchase Ignition Bombs, booby-traps which explode when a Vehicle's key is turned. Watch out for suspicious-looking parked cars. Even more insidious are Remote Bombs, remote-detonated explosives designed to punish carjackers.
You don't want to spend your entire time online with nothing but a Pistol, do you?

Complete Optional Mission Challenges

It pays to complete every Mission to perfection. Optional Mission challenges add to your Reputation. And the better your rep, the better your online experience.

Join a Crew

Crew membership likewise adds to your Reputation. To max out your RPs, team up with a Crew and work together to complete Jobs.

Character Creation

IGN has a full Guide to Character Creation available for new players, and we encourage you to read it since the Stats determination system is pretty opaque. Peruse the Guide and carefully select what abilities you want your new character to specialize in. 
Facial features are permanent and unchangeable, so when fooling around with the Heritage menus, be absolutely sure you're happy with your face. Barring the unannounced introduction of plastic surgery clinic, you're stuck with it!

Phone Contacts

Your cell phone works online, and you're able to communicate with other players through your Contacts list.
Details on this are still unclear, but we'll be adding them as we learn more.

Winning the First Race

Lamar's car accelerates more quickly than yours, but you can outrun and out turn him given time.

Indecent Behavior Leads to Indecent Treatment

Early word from those with GTA Online access is that in-game protocol is creating a self-correcting system of player discipline. Campers, griefers, and jerks tend to be ostracized quickly in the sixteen-player maps, with more level-minded players bonding together to keep the playing field fun for all...not necessarily peaceful, but fun.
Protocol largely seems to center around spawn-camping near Hospitals. Griefers have been reported hovering near these community respawn points killing characters as they return to life.

Bank Immediately

You're vulnerable after you finish a Job. If you don't deposit your cash immediately, you're open to robbery by opportunistic players.
Protect yourself from theft by using the Quick GPS feature. You bring it up with the Back or Select key. It's faster than the minimap and can get you to an ATM with precious seconds to spare. Or simply go online on your in-game phone and deposit your cash through your bank.

Buy a Garage

After an SMG, a Garage might be the wisest investment you can make in the early game. With a Garage you can begin hoarding Vehicles and putting your newfound online wealth to work.
Garages are pricey and you'll need to save up, but the investment is well worth the cash. Even the cheapest Garage is a wise day one purchase.

Go Solo

If you want to experience GTA Online but don't want to spend time worrying about getting killed by other players, going to solo mode in the pause menu can be a good choice. You don't get access to all available jobs and activities, but it can be a good way to get adjusted and a get few dollars in your pocket before joining everyone else.