Can you be forced to get a transplant?

Depends.  If you're opposed on religious grounds "or" if you have documentation stating that under no circumstance are you to have a transplant (with someone able to execute this) or a DNR, then you're probably okay.

There is a long waiting list for transplants.

It is unlikely they would waste one on a non-compliant patient.

No, not at all. If you needed a transplant you would be extremely lucky to actually get one. Viable and compatible organs aren't that common in the grand scheme of things. If you opt out of the chance to receive a transplant you can bet it'll be offered to the next eligible patient almost immediately.

can a patient be forced to receive an organ?.. maybe if he/she is a child below the age of comprehension, less than 10 or 12 years old.. otherwise, the answer is definitely a no.. 

transplant recipients usually undergo a strict interview and assessment by doctors.. if the doctor(s) get(s) any hint of suicidal-thinking or self-harm in an organ recipient, it would be an automatic disqualification to receive an important organ, such as the liver, even if the patient is 13 years old or below the age of consent in most countries when his/her body is legally under the parents' rights..  

getting a donor liver is a difficult task, it can't be wasted on someone who may self-harm him or herself..