Well first thing I see the lines
// Display tax
Console.Write("$");
Console.Write(dectax);
// Keep window open
Console.ReadLine();
Should be placed outside the last if block because you'll want them displayed every time. The next thing I see is that it looks like you're doing the math wrong. From my limited knowledge of income tax and how it's graduated it works on blocks of income.
Example:
A guy makes $60,000.
On the portion of his income above $38,000 he pays 33% income tax, or about $7,260
On the first $38,000 he pays 19.5%, or about $7,410
Meaning in total he pays $7,260 + $7,410, or $14,670
So the math looks likes
((60,000 - 38,000) * 0.33) + ((38,000) * 0.195))