What can decrease my chances of getting pregnant for ex I smoke weed & so does my bf usually on a daily basis no tobacco. Way can increase mychances to?

Consider quitting. If you and your boyfriend are trying to conceive, you should both consider quitting marijuana. Not only does it lower his sperm count (making pregnancy less likely), but once the baby comes you both need to be sober in order to be the best parents you can be. See your doctor for resources and support to help you both quit.
Weed. Marijuana has been associated with dna breaks to a greater extent than tobacco. The resultant munchies can be associated with weight gain and can abnormal menses. I'm not familiar with the literature on breast feeding but don't think you want toexpose a baby or fetus. Id suggest you figure out how to cut this out for now.
Many problems. Mj is associated with increased risk of chromosomal damage, prematurity and later learning and behavior problems in fetuses exposed to it. Exposure to mj prior to adulthood leads to decreases in iq. A responsible adult person would discontinue mj, alcohol and tobacco before even considering conceiving. Since you appear ready to risk it, I suggest you may not be ready to be parents.