FSA Compact 50/34 Need better climbing
The largest cassette you can use is determined by the rear derailleur (in particular, its chain wrap capacity). For something larger than 30t in the back, you want a wide range derailleur (code SGS) - something like a Deore rear derailleur if you're running Shimano (at 9 speed, the mountain and road rear derailleurs can be swapped, and the mountain rear derailleurs can take bigger cogs in the back). Then, put on some bigger cassette which works for you - your local bike shop will probably know some cassettes which work well in your area.