It is difficult to respond without providing at least some code. See the documentation for the stem (link) function for details.
For your first plot, try this:
x = [0 2 3 2 1 -1];
figure(1)
stem(x, 'filled')
set(gca, 'XLim',[0 numel(x)+1])
Ideally, you should also have an independent variable vector, however if you do not supply one, MATLAB will plot them sequentially with a ‘step’ of 1.
