Don Corleone wanted nothing to do with the drug trade. As is apparent from this meeting. When Sonny spoke up to Sollozzo and says,
"Now wait a minute. You're telling me you can guarantee...?"
The way Sonny delivered the line and his actions while doing clearly indicate that Sonny (and maybe the Corleones) are interested in the proposal, seeing as he spoke up before the Don. Also this also portrays to Sollozzo that if Don Corleone were eliminated and Sonny were to take over, then Sonny may accept his offer.
Vito Corleone refused, however, feeling that the drug business is bad for the neighborhoods. However, in the course of the meeting, Sonny Corleone admitted an interest, leading the Turk to think that if Vito died, Sonny would accept his deal.
Basically the motive for Sollozzo from this point on in the film is to get rid of Don Corleone and use Sonny's apparent interest in the heroin trade to solidify a bond between the Corleones and Tattaglias (aka the rest of the story).
This spontaneity of emotions from Sonny tells Sollozzo exactly what he's thinking at that given moment, which in poker terms is "folding your hand." Meetings between two mob bosses are basically a high stakes game of poker to see who folds first. Think of it as two opposing forces.
When Sonny "put all his cards on the table", Sollozzo basically came out on top. The difference between Sonny (the hot head) and Michael (the calm vet) is part of the reason one survived and one didn't.
Don Corleone telling Sonny to,
Never tell anyone what he was thinking.
is a way of saying "Don't be so upfront." It may seem like Sonny didn't tell Sollozzo much with just half a sentence, but in reality he told him a lot.