1. Go to 'Search' -> 'Contributions' in your menu bar, and using the 'Contribution Dates:' fields, find all the people who gave during your last fiscal year.
2. Go again to 'Search' -> 'Contributions'. This time, make sure that 'SELECTION MODE:' is set to 'Filter'. Now, using the 'Contribution Dates:' fields, set 'Beginning Date:' to the beginning of your current fiscal year. Click 'Find'.