You can apply conditional formatting to your sheet that will allow you to format every other row however you specify.
Select the area where you want to highlight alternating rows.
Go to the Format menu and select Conditional Formatting.
For Condition 1, select “Formula Is”, and type in “=MOD(ROW(),2)=0” if you want to highlight every even row, or type “=MOD(ROW(),2)=1” if you want to highlight every odd row.
Click on the Format button, and choose the format you want your highlighted rows to have (i.e. font, color, background, patterns, etc.).
Click OK, and you should have every other row highlighted.