Buy a roll of kraft paper, make sure it is 4-6 inches larger in length and width than your print. Fold up an inch or two along the bottom, this makes a pocket at the bottom in which to place the print. Many people will now put tissue paper to cover the lower third of the print in order to protect the surface of the print from the edge of the paper once you start rolling. Roll up the print, don't press too hard and make sure you are rolling it up straight and not at an angle. You want the roll to be smaller than the tube, but not so small that it rattles around. Make sure the kraft paper completely covers the edge of the print and tape down the edge of the kraft paper. I usually fold a corner of the tape to make a tab that allows the person to easily take it off (stole that from Mondo). Usually three, three inch long pieces of tape will secure a 24" x 36" poster. For tubes, Yazoo is the most popular but just make sure whatever you get has thick walls or, for expensive stuff, people will often use PVC pipe. If the roll is shorter than the tube, to the point that it slides back and forth inside the tube, you can pad each end with wadded up tissue paper. Tape the shit out of the end caps. I usually cross over the ends in a star pattern until the end cap is covered then wrap tape a few time around the end of those pieces below the cap.
Shipping service is a matter of opinion and region. I use UPS usually but many people hate them and have had bad experiences where they live. Many prefer USPS which, conversely, has been the only shipping service to deliver me jacked tubes so it's a bit of a crapshoot. Just make sure to insure it no matter how you ship.
Hope this helps, my apologies if you knew some of this already. As I said before it's not super hard, just take your time and you'll be good to go.
