What are the nucleotides? Which pair with which during DNA replication and then from DNA to mRNA?

The DNA nucleotides are as follows:
Adenine (A)
Guanine (G)
Cytosine (C)
Thymine (T)
Adenine and Guanine are 'purines', which you can just think of as being larger. Cytosine and Thymine are 'pyrimidines', which are smaller. Larger always pairs with smaller, and specifically, A  always bonds with T, and G always bonds with C during DNA replication.
The only difference between this and going from DNA to mRNA is that with RNA, instead of thymine (T) bonding with A, uracil (U) always takes its place.

1. T-G-A-G-G-A-C-T-C-C-T-C-T-T-C
2. assuming we are transcribing from the original sequence:
    U-G-A-G-G-A-C-U-C-C-U-C-U-U-C
3. when translating the mRNA to amino acids, every three RNA nucleotides (called a 'codon') codes for one amino acid.
A chart that matches codons with amino acids can be found here:

http://en.wikipedia.org/wiki/Codon