This is basically an unsolvable problem. You can't make it impossible for someone to change the parameters sent, but you can make it more difficult.
Perhaps the easiest way to stop people from modifying the query is to have a secret code built into the game, say for example "34fE_ef" and encrypt the sent data with that key. The server can then decrypt the data using the same key.
This isn't foolproof of course, someone would be able to search the source and find the key. So you could make the key harder to find. Doing something like this should stop most casual hack attempts.
Another way is to do sensibility checks, the difficulty/effectiveness of these checks is very dependant on the type of game you have. A sensibility check basically says "the submitted score/position/whatever of this player is impossible to obtain so I will ignore this".
Just some ideas anyway!