I prefer a thickness of 0.75mm, but other thicknesses will also work. Just remember not to get anything too thick. Remember that when soldering, the rosin in the solder releases fumes. These fumes are harmful to your eyes and lungs. Therefore, always work in a well ventilated area.
Hot solder is also dangerous.
Be sure not to let is splash around because it will burn you almost instantly. Eye protection is also advised. All surfaces to be soldered should be cleaned with steel wool and some sort of solvent. Laquer thinner works well. Some people like to use sand paper, but I think that it is all too easy to sand right through circuit board traces, so steel wool is my preference.