Genetic conundrum +20 characters?

I decided to get all of my genetic questions out of the way; if you just know one of them answer anyway please and thank you! How do genes turn off and on? If an individual is heterozygous for a particular gene, are both genes used for protein synthesis or just the dominant one? What decides which allele is dominant? If a lot of our genome is just random **** that doesn't code for anything, how do cells know where the randomness stops and a gene starts?