You can use 3/8" rebar. Buy it or cut it into about 2 feet long sections (buying longer and cutting yourself will be cheaper).
Push about 1 foot of the stick (for lack of a better term) into the ground, leaving 1 foot exposed. The electrical conduit or PVC pipe, which should be hollow, can now fit on top of the 1 foot exposed section. Repeat this on the other side of your garden bed or a few feet over, depending on how wide you need your trellis to be.
(This would give you 2 vertical pipes and then connect the horizontal one).
I have also seen people go as much as 4-foot sections, with 2 feet in the ground. That should just give it more stability, depending on your windy/stormy climate or weight of the veggies you plan to support.
Now that you have the basic frame up, just pick the kind of material you want for a trellis "netting" that goes inside the frame.
