For expression analysis, you can use the Oncomine website and Database :
http://www.oncomine.org
(registration is free for academic users)
where you can search for a specific gene and look at its expression in a vast series of expression data. You can filter the data, for example restricting the analysis of mutations in primary samples and not cell lines, or for certain types of cancers.
For known mutations in cancers, you can use COSMIC : Catalog of Somatic Mutations in Cancer, there : http://cancer.sanger.ac.uk/cancergenome/projects/cosmic/
where you can search by gene name, and also filter the result by tumour type, tissue, etc...