It all depends on how much you are talking about. A small amount 1/8 inch of twist or warp is removed best by the jointer. That is the main use for a jointer. You run the face through the jointer until it is flat, Place the face on the fence and run an edge through to get a true edge at 90 to the flat face. You can then rip to width and plan to at true square board.
If you have a lot more than an 1/8 you can still do the same but you will loose a lot of material. I would cut the piece down to shorter lenghs then you will have a smaller twist to take out.
You can do the same with hand planes but it take skill and time. start by takeing off the high spots on the face and keep planning until you have a flat edge using a jointer plane. the use a shooting board, joint the edge. Using good measurement gauges plane the other face paralell and true to the first face and the true edge.
What do you mean "wavy"? Lengthwise curve (curved)? Bowing (curved to the face)? Cupped (curved edge to edge - very common)? or Twisted (end to end twist)?
If the wood is curved severely there are two ways to get rid of it. First you can use the crosscut saw and lop that piece off the end. If you cut where the curve starts you'll get two reasonably straight pieces that you can use the joiner on.
If lopping the curve off is not an option you can rapidly straighten out a board on the TS. Attach a straight "carrier" board across the tips of the curve (board center projects away from carrier. Then use carrier board as straight reference and rip hump from center. Remove carrier and flip board over and use just cut edge as reference and take off tips and just enough to skin the center. You'll now have a straight board with curved grain. Clean up on jointer if you have one or not if your saw does a good job.
Bowed lumber can be difficult. Again first solution is to chop it down into shorter lengths for manageability. Jointer can take out a lot. Planer can take out some.
Cupping/crowning is handled by the planer with ease.
Twisted is best handled by using it for kindling.
Generally I don't *buy* lumber that is severely cranked.
I BUY MY HARDWOODS IN BULK FROM A SAWYER/KILNDRY OUTLET.
BUYING A WHOLE TREE ,THUS GETTING ALL GRADES INCLUDING SOME
BOARDS THAT ARE WARPED OR HAVE A TWIST.
BEING FROM THE SELF TOUGHT SCHOOL, I FIND THAT A CHALK LINE
SNAPPED ON A WARPED BOARD GIVE A PERFECT LINE WHICH I WALK THRU
THE TABLE SAW. TURN IT OVER AND THE USE THE FENCE FOR THE NEXT
CUT. THE JOINTER WILL FOLLOW TO TRUE THE EDGES.
AS FAR AS A TWIST, IF IT IS REASONABLE, I RIP THE BOARDS INTO PIECES 2 1/2IN.OR SO AND REGLUE THE IN APPOSING GRAIN. BY
DOING THIS IN MANAGABLE LENGTHS AND USING C-CLAMPS AT THE ENDS
THE RESULT AFTER PLANING GIVES A STRAIGHT FLAT BOARD.
I HAVE DONE THIS FOR YEARS AND LONG AGO DISCOVERED THAT SCRAP
WOOD COST THE SAME AS THE GOOD.
...........freehand a cut through a table saw. The best way to straighten a board is to clamp or nail a straight edge on top to run against the saw fence. That will give you one good edge to work off of. If there is cupping,rip the board at the lowest point in the cup,turn one half over and re-glue the joint,then surface plane off that.
Roger is very right, but he didn't say why. You are almost guaranteed to get a real bad kickback from a freehand cut. The back of the blade is moving up and a very fast speed. If you twist the board a little and it catches it will be coming back at you as fast as 100mph. Not good. Can be very very bad. It is things like this that people will try to stop the board and end up with out fingers.
One former friend lost 3 fingers on his left hand by doing just that.
Use your tools safely. Use hold downs and feather boards. Always use a fence for rips and not for cross cuts. Keep out of the way of potential kickbacks and keep your hand far from the blade. If the stock does take off stay out of the way.