The goal is to plant (usually) that the most profitable herb seed that RS gold will leave the most profit when harvested. I am not sure on the best level to begin, or even the current pricing. However, I likely would not suggest you doing it with your level until you unlock some more herb choices.
Doing Fairytale part 1 is a great advantage, as the secateurs will enhance the number of herbs accumulated. Another good idea if looking to do a little bit of herb runs in the long run is to invest in the Herb Scroll from Dungeoneering, since it will provide some potential"money back" if you are fortunate. Additionally, doing Ghosts Ahoy! Can assist you by providing the ectophile for an easy teleport into the swamp patch. Doing the lumbridge/draynor acheivments will assist you with an easy teleport to the draynor patch.
When farming, it's also wise to utilize supercompost EVERY TIME. Also, to clear up any misconceptions, you cannot cover a farmer to watch your herbs, you can only pay him to watch the fruit and Buy old school runescape gold vegetable plots in the allotment.