How do I build bitcoin transactions graph? I'm using blockchain dumps from http://dumps.webbtc.com/ 
in database schema in mentioned that result of script in pk_script should specify recipient of transaction. But how do I get amount of bitcoin sent and sender?
The database schema is located here: https://webbtc.com/api/schema

You might be disappointed to find out that it's not as simple as an amount and a sender for each transaction. What there are is a list of inputs (references to previous outputs), and a list of new outputs. Each output with an amount and a pseudonymous address. Each input has a TXID and output index to reference which previous output it is spending. Unless you know who owns specific addresses, though, you can't determine the identity of the sender(s).

You can use the prev_out and prev_out_index from a row in the inputs table to find the associated previous output, which will tell you the amount and address.