I did the exact same coursework in AS level are you told how to do it? I can't remember if we were. Basically its a case of looking at the stoichiometry of the equation so for the titratration one its going to depend on the equation (don't know what you're titrating it with) for the adding it to water its
Na +H2O ----NaOH +0.5H2
so you find the mass of Lithium you're using measure the volume of H2 given off (we used gas syringes I don't like the wet method with an upside down measuring cylinder) and work backwards to find the number of moles of gas to find the number of moles of Li (Twice as many) to then work out the relative molecular mass. You can predict the amount of H2 from working forwards using the relative mass off a periodic table. Have fun