Grignard reactions with an aldehyde will form a primary alcohol; reactions with ketones will form a secondary alcohol.
Grignard reagents, aside from being great nucleophiles, are also strongly basic. Therefore, if the solvent is polar protic, like water, then that will get deprotonated and the reaction will fail. So, your solvent should be aprotic, like diethyl ether.
The mechanism is pretty simple. Simply put, the electrons from the partially negative carbon will attack the carbonyl carbon, forming the alkoxide. This is followed by an acidic work-up to form the alcohol. Note: you would allow for the grignard to react with the carbonyl compound FIRST before adding any acid (see note above).