Question

hoe is it decided which trait will be inherited and character will not be inherited

Answer 1

Inheritance of Traits by Offspring Follows Predictable Rules. Genes come in different varieties, called alleles. Somatic cells contain two alleles for every gene, with one allele provided by each parent of an organism.

Answer 2

Suppose, father have bb eyes and mother has ss eyes. b is for black and s is for blue. Then offspring will have bs genotype but only the black eyes as black is dominant gene while blue is recessive gene.
Also, for monohybrid and dyhybrid crosses, you can use Punnet's square to determine traits. I will recommend you to watch video of Amoeba Sisters on youtube regarding Punnet's square.