Choosing a FPGA

If you mean logic blocks, these are the basic logic elements in the chip - consisting of a LUT (to create the logic equations) and a register.

DSPs are basically multipliers

Memory blocks are self explanitory.

If you dont understand these, then you highly suggest you get learning about digital logic and how it works.

Only you can determin how many you need - you should be able to estimate DSP and Ram requirements from your algorithm.

Without understanding the basic elements, you have no chance choosing what board you need.