Being overweight has nothing to do with erectile dysfunction. However, there are other health issues that could cause this problem in a young dude. You should get a general physical exam by an internist or family practice doctor, which should include a fasting blood glucose test.
Physical causes of ed in young men:
1) diabetes -- diabetes can cause call kinds of vascular problems.
2) hypertension medications
3) antidepressant medication(s)
4) spinal cord/neurological injuries
psychological causes:
1) anxiety
2) depression
what's your health like? Can you get a firm erection on your own, or is the problem only with your girlfriend/wife? You should get an exam to make sure you're okay. You don't need a urologist now, because he/she will want a complete physical exam before you see him/her, so see an internist and get some blood tests. Rule everything out before you seek treatment.