A friend told me that I have to use a bandsaw for this task but what if you only have a table saw?
Bandsawing has become the go-to method for resawing in modern times, and purpose-made blades are available for resawing. The bandsaw offers numerous advantages for the task: pressure is downwards, into the table, so there's no chance of kickback throwing the wood at the operator; cut depth can be large to very large; the kerf of bandsaw blades is significantly narrower than that of most rotary saw blades so you waste less wood1.
But not everyone has a bandsaw of course and those who own a table saw will eventually consider doing it on that. As a result there is a lot of guidance to be found online on resawing on the table saw.
Here are what I consider the basics in bullet-point form:
Resawing is a rip cut and as such a rip blade is most appropriate on paper, but admittedly many people just use their general-purpose blade they use for just about every task. Thin-kerf blades will offer significantly less resistance when resawing thick and/or hard stock on lower-power table saws.
Let the saw do the work. Don't force the wood through the saw, especially on deeper cuts and/or with harder species, just let the saw cut at its own pace. Sawing at a measured pace means you may inevitably leave some burn marks on one or both of the final boards, but the sawn surfaces need a fair amount of cleanup anyway so you're not adding unnecessarily to the post-sawing workload if slowing down yields more burns.
Use your riving knife or splitter. If your saw doesn't have one, make one (see below).
Vital to use a zero-clearance insert if sawing thin stock. If your saw doesn't have one, make one! If your saw doesn't have an in-built splitter take the opportunity to add one.
Use a tall fence; fit an auxiliary board to your existing fence if necessary. The fence does not have to be taller than the stock being sawn, but it should be tall enough that it provides good registration for the height of the board upright on its edge.
Use one or more featherboards or other pressure devices to keep the stock against the fence, helping to yield an even cut. This can be as simple as a board clamped to the bed of the saw as long as you make sure it doesn't pinch the workpiece on the exit side.
Before you resaw it's important to prep your wood. Ideally all wood to be resawn should be four-squared already. And it's often said that the wood being processed must be all the same thickness, but while this is desirable it isn't absolutely necessary — one face is run against the fence even with boards too wide to split with a single pass, so it is the spacing from fence to blade which ensures the keeper piece will be a consistent thickness, regardless of the thickness of the starting boards (just remember that adjustment of pressure devices might be needed from one board to the next).
Use push sticks or push blocks to control the board and provide downward pressure on the stock as it passes over the blade.
To echo a point made above in the Comments, you'll have no difficulty finding people resawing on the table saw who aren't following some or many these guidelines :-(
Some links for further info and to see the process in action.
Resawing at the Tablesaw; It’s often the perfect tool for the job on Woodcraft Magazine's site (PDF).
Use your table saw instead of resawing with your bandsaw on WWGOA website.
Resawing on the Table Saw from Tage Frid Teaches Woodworking, vol 1.
Safest way to Resaw on a Tablesaw by A Canadian Woodworker.
Resawing On Your SawStop Table Saw from SawStop.
Honourable mention for resawing by hand, at least sometimes
As most guides show, if your board is too wide you need to saw through the 'web' in between the two saw channels. Although paradoxically some use a bandsaw for this the standard way this is shown is to cut it with a hand saw. This gives an entre to doing the whole task manually — with a suitable saw2 it isn't so much effort that you should dismiss doing it that way, except in very hard wood and very wide boards.
Being able to resaw by hand is a useful skill to have for when the table saw it out of action, the power is out and you need to get the job done or you're somewhere where there isn't any electricity. Or maybe you just need to do it quietly so as not to wake the neighbours or family!
See page in Tage Frid's book immediately above the one linked to above for some starter advice on doing this. There is now lots of guidance on this online now with the resurgence in interest in hand-tool woodworking for further tips and tricks if interested.
1 This may not seem that important if splitting e.g. an 8/4 (50mm) board into two and you're happy to get two 3/4" (18mm) boards out of it, but it can be a vital consideration when starting with thin stock and you want the final pieces to be as thick as possible.
2 A Continental frame saw is arguably the best tool for the job, although it can be done with efficiency and accuracy with a traditional English or American rip saw with a low tooth count. Although not made specifically for ripping one of the better modern panel saws with impulse-hardened teeth (e.g the Predators from S&J) can do the job reasonably efficiently and are far easier to acquire, plus they come ready to use and super-sharp.