Before you dye your 100 % cotton or wool garment, (on blends, the dye might not take so well- I advise that you test a scrap piece before you dye the whole garment) you should mordant it so that it will accept the dye better. I also suggest that you wait until the garment is finished to dye it, rather than dying the fabric. This way, if you use cotton thread, the thread dyes to match the fabric exactly. Also, it means less fabric to have to stuff in the pot.
Goldenrod makes a lovely yellow dye on cotton (alum mordant), or a yellow-brown on wool with a chrome mordant. Most reference books recomend that Goldenrod isn't extreamly colorfast on cotton, but I've had good experiences with it- as long as in the future you hand wash the fabric in cold water with very little soap. Besides, if the color fades you can always re-dye it (especially since the ingredients are weeds).
First you will need to gather about a one and a half gallons of just-opened goldenrod heads (I've used ones that had started to go to seed before- but the color wasn't as strong). Click here for a photo of goldenrod, if you're not sure what to look for. This plant, in some form or another, should be available in most of the United States, and blooms from July to September. Make sure to pick only the heads.
To prepare the dye, cover the flowers in 2 or 3 gallons of water, and boil at a medium heat for at least an hour. Test the dye repeatedly with strips of cotton (you don't need to mordant- it's just a test) to check if it's strong enough. When the dye is suitibly strong, strain it well (through cheesecloth), and imerse the fabric. When garment has reached the desired color (remember it'll be a little darker now becuase its wet), squeeze out and lay on a towel to dry. When the garment is fully dry (allows the color to set), you can wash it if you find the odor unpleasent (a note to hayfever sufferers - goldenrod is not ragweed, it's just maligned becuase it is the same color and blooms at the same time). Or, if the fabric is cotton, and you're worried about fading, you could throw it in the dryer with a dryer sheet.