You can pass the format in to the to string method
eg:
ToString("0.00"); //2dp Number
ToString("n2"); // 2dp Number
ToString("c2"); // 2dp currency
http://msdn.microsoft.com/en-us/library/dwhawy9k.aspx
To change, for example, 13.505 to 13.00 you'd also want to run it through Math.Floor or use one of the other suggested methods for rounding down.
Math.Floor(value)
http://msdn.microsoft.com/en-us/library/e0b5f0xb.aspx
If, on the other hand, you want to change 13.505 to 13.50 you'd want to run it through Math.Truncate.
Math.Truncate(Value)
http://msdn.microsoft.com/en-us/library/7d101hyf(v=vs.110).aspx
So to tie that together:
Double testValue = 13.505;
Double testValueTruncated = Math.Truncate(100 * testValue) / 100;
string withDecimalPlaces = testValueTruncated.ToString("0.00");
withDecimalPlaces will now have the value "13.50"