i have 2 yrs data in my database can i backup last yr data only??
if possible
Not unless it's already in a specific file, I don't think: http://technet.microsoft.com/en-us/library/ms179316%28v=sql.105%29.aspx
whenever i want full database can i combine those 2 backup??
Which two backups? You would just carry out a full backup if you wanted to back up the full database.
how can i get full database??
It's not clear what you mean.
Before you go any further you should try to read this book http://www.sqlservercentral.com/articles/books/89519/ (the e-book/pdf version is free of charge)