Generally you have to have a good degree of fitness, when going for medical you may have to demonstrate the ability to lift objects over your head, balance is important, in training you may have to lift emergency exits/lift rafts/other equipment etc some of which can be quite heavy.
In Australia you can not consume alcohol 8hrs prior to sign on and drugs are no-no's. Companies are trying to incorporate random drug & alcohol testing so you do need to be careful.
You must wait 12-24hr before flying after scuba diving depending on what type of dive you did.
After giving blood a healthy person should wait for 24hrs before flying. For some types of blood donations 48hrs is recommended.
There's plenty of information on teh web if you do a search though.