You can try to look for your protein on the Uniprot web site (http://www.uniprot.org/) by using either the name of the protein or by using its accession number ( if this protein have been already sequenced).
Once done you can access a number of structural information that includes: full sequence, number of AA, molecular mass, theoretical pI, C an N terminal sequences....These features are easily accessible by using a drop down menu in the section sequence.
If you do not have the accession number you may try to conduct your research first on PubMed to get the ID of your protein or directly select from the results options the structural features you are looking for.
Both Ibtissam and Andrei give links to excellent on-line tools where you can calculate the pI of proteins, but if your protein has post-translational modifications then the calculated pI will be wrong.
Calculating the pI from the sequence is always going to be an approximation. If there are ionisable groups which are buried within the structure, the pI of these groups may be influenced considerably by the local environment of that residue. So the above tools give a reasonable approximation, but caveat emptor.
Hi! Colleagues are absolutely right, it is very useful to estimate pI of your protein, and it is easily done by many programs both online and standalone. However, since it is done solely from the primary structure and does not take into account many real factors, this pI value should be considered as a guidance only. Experimental pI should be checked for particular cases specifically. Also, try to check literature on pI values of your stuff.
Andrei is right. Theoretical pI obtained from the amino acid sequence can easily be obtained at expasy and other web services, but most of the proteins have post-traduccional modifications and experimental pI is different of the theoretical one. Several isoforms with different pI are also common. It depends on the protein and the source. Cheers.