Here's a quicky. I got this idea off the internet and this is my version.
It's cheap and quick!
Step 1: Select Your Location
First thing to do is select a suitable location for your bed. This may vary depending on what you are going to grow but for veggies generally the more sun the better.
Step 2: Materials
3-cedar fence slats from big box store for a couple of bucks each
Scrap lumber for corner blocks and stakes
Exterior wood glue
Step 3: Prepping
Start by cutting one fence slat in half so you end up with two six foot boards and two three foot boards.
Next prepare corner boards by cutting 2X2s to a length that is equal to the width of the fence boards you purchased.
You will also need about four stakes that are about 6" longer than the width of your fence boards.
Now clamp a corner board to each end of the 6' pieces, pre-drill and then screw the fence board to the corner board.
Step 4: Attach Sides and Ends Together
Once you have fastened the corner blocks to the side boards. Pre-drill and screw the ends to the sides. You should end up with a frame that is approximately 3' by 6'
Step 5: Locate
Next place the frame on the ground in the location that you previously selected. Drive a stake in each corner to keep it from shifting around.
You may need to place a couple of stakes on the outside in the center of the long boards to keep the soil from bowing it out.
Kill the grass/weeds etc. in the planting area. You could also turn this area with a shovel and remove the vegetation.
Fill your box with soil and plant your veggies.