It's generally good advice to only pick stuff you can confidently identify. If you have any doubt, leave it. It's also good to avoid stuff that grows too close to the sides of roads, as they may contain more toxins. And wash everything before you consume it.
I took the picture above on Barnes Common, an area of west London which has remained beautifully underdeveloped on account of the land's marshy, flood-prone quality. While it has little value for property developers it maintains great value to the local ecology. The main plant in the photo is a dandelion, which has just begun to flower.
Now, its generally best to pick dandelion leaves before it flowers, as afterwards it can develop a more bitter flavour. Some people boil them for about 5-10 minutes to reduce the bitterness of raw dandelion leaves. The leaves make a nice peppery addition to salads, while the flowers, as rich in vitamin A and C as the leaves, can form the basis of herbal teas and even be brewed into dandelion wine. Later in the year the roots can be dug up, washed, dried, roasted and then grated up to create a caffeine-free coffee substitute. At present I mostly use the leaves.
I understand that if you take the fresh root as a tea it works particularly well for detoxifying the purifying the liver, kidneys, gall bladder and regular bladder.
The other plant you may be able to see in the photo are the long, thin leaves of the cleaver (or goosegrass), which is also called "sticky willy" because it has a velcro-like texture which probably helps it cling to the fur or feathers of birds and other animals. You can chew it fresh or drink it in hot or cold infusions, and it has a pleasantly refreshing flavour, as well as providing a rich source of natural vitamin C, as well as the same cleansing properties as dandelion. The seeds of the cleaver provide a coffee substitute.
The value of the wild plants that pop up in springtime are as cleansing, detoxifying agents, which means you may have to visit the toilet soon after drinking, so make sure you take precautions. This is why the ancient folk name for the dandelion was "piss-a-bed."
Some cleavers I picked during a wild food walk in Sipson