NCBI is nice. But I also like the website http://www.expasy.org/
Beside the Swiss-Prot and TrEMBL protein databases, you can also find a lot of links to different other interesting sites, tools and documentations. The Swiss-Prot database has, to my opinion, a very nice data format. If a protein is well characterized, you can find the complete precursor sequence and easily get the information which part of the sequence belongs to the mature protein, sequence peptides and propeptides. Known mutations and PTMs are given as well.
To get more detailed information, it is always good to have a look into the online-documentation of the databases you are using. In addition, there are some nice tutorials in the web, which are free. See for instance http://www.bioinformatik.de/cgi-bin/browse/Catalog/Research_and_Education/Online_Courses_and_Tutorials/
Regards,
m/z