A tripod allows you to shoot in low light at low ISOs with long shutter speeds.
Your camera can see far into the distance when anchored securely, far beyond the the ability of any flash to illuminate.
When using a tripod, you are fairly well forced to slow down and really think through your composition.
A tripod can be used in a studio setting for products and models, awesome for landscapes, pretty darn good for portraits. Pretty much essential for steadying telephoto lenses.
They can be useful during weddings, certainly not for all situations.
Not so good in stadiums, anywhere there are moving crowds.
Can be useful for street photography though one often wants more mobility that you'll have with a 'pod.
If your budget is tight, look at Manfrottos with aluminum legs. Big budget, go carbon fiber, there are many good names.
Tripod head is a separate part. Your composition should not sag down as the head is locked.
Really cheap tripods are among the worst possible investments, I'm thinking $150 to $180 USD as a bottom starting point, creep or leap upwards from there as budget allows. I'm just about ready to leap to a set up I'm fairly certain will excel at every task I can imagine throwing at it and may well last a lifetime.
All else being the same, three section legs will be stiffer than four sections, four sections will fold smaller, I'm sticking with three.
About legs, look here...https://www.google.com/#q=moose%20peterson%20tripod&safe=off
For heads, start by browsing here.......http://www.bhphotovideo.com/c/buy/Tripod-Heads/ci/140/N/4075788769
then start asking around.
One strong recommendation is a head with a quick release base and a corresponding plate for your camera. Do consider an Arca-Swiss compatible QR configuration, not quite universal but it should be.
Heads and QR run from inexpensive to WOW, really?.
Filters, yes, I likes me a polarizer, keep two in my belt pack. NDs and ND grads are on my want/wish list.
Flash, eventually. Several. And remotes.
Pack comes later, when you've got a better idea what will be going into it. You'll eventually have many.
External drive, depends on your current computer's current hard drive free space and if you've got other back up options like DVD burning available. Eventually, many, preferably some off site storage as well.
Straps are good, most use them. Sometimes one of the more controversial topics here at CR, you try and you decide.