what is this plant?
Your plant is an annual weed called prickly lettuce, Lactuca serriola. It is almost ready to flower. When the seeds are ripe they will blow all over your yard, garden and neighborhood.
It's easy to eradicate. Put on gloves to protect your hand from the spines on the leaves, grasp the stem just above the ground, and pull it out. You can compost the plant except that it's bests to break off the buds and discard them.
See "Prickly lettuce" (http://ipm.ucanr.edu/PMG/WEEDS/prickly_lettuce.html)