Null values In R.

Hi everyone.

I am having problems with NULL values. I understand in R one can
command the program to skip null values.
Can some one help me on the command line for that.
Do i enter is as part of the string in:
a<- read.table("filename.csv", header = T, row.names=1, sep=",");

My problem is largely when i attempt to use correlation for my data...
xcc <- cor(a);

The  error i get is as follows

Error in cor(a) : missing observations in cov/cor
In addition: Warning message:
In cor(a) : NAs introduced by coercion


Regards

Paul Murima