There is a big fish at your table, now how do you reel him in? Obviously there are different types of weak players but the atypical fish is the guy that has a hard time folding...anything. It's hard to fight the urge, but the number one rule to go by is to not go nuts and try and bluff them out of the pot. Patience is one of the key skills against weak players, because they don't have it themselves, so don't try and win it all at once.
Wait until you are dealt a good hand and when you are, charge the maximum for it. Most of the time players will want to size up their bets as a percentage of the overall pot, ie. half the pot, ¾ pot etc, but if you think the player is bad enough, by all means overbet the pot or even just go all in. If you think they are bad enough to call you with a worse hand, go for it.
Against bad players you can get away with value betting much thinner than against anyone else. A thin value bet is when you think there is a very narrow range of hands you beat that your opponent will call with, but it's still likely they would call. You might not dream of betting bottom pair on a coordinated flop against most people, but the range that a fishy player has is so much wider, it will be conceivable they could call you with worse.
There will be times your fishy friend will outdraw you in a big pot and take your money, do not let this get to you. It's a good thing, in the long term, that he can get lucky and win a few because it keeps him in the game longer. Resist the urge to tell him what you think of him in the chatbox, if anything congratulate him and make the experience more fun for him, because it will pay off dividends later on down the line.