We can aboard the points of the question from the first one to the last one.
1. Regarding the similarities between RNA and DNA, it can be said that they share Adenine, Cytosine, Guanine, and Thymine as nitrogenous bases and that both are nucleic acids.
2. Regarding the differences, we can say that DNA is a double-stranded molecule, while RNA is single-stranded. Also, we can say that RNA has a specific function of transcribing the information into proteins, while DNA is the mold of RNA to do that function. And regarding its nitrogenous bases, in that transcription process, RNA replaces Thymine for a base not found in DNA: Uracile.
3. In DNA, what pairs with Thymine (represented with green color in the DNA molecule of the draw), as we can also see in the draw of the picture, is Adenine (represented with red color in the DNA molecule of the draw).