What you want to do is increase the TOTAL diameter of the wheel and tire combo.
Assuming your tire size is 225/45/16:
Tires on your stock 16s could go up a size or 2 to 225/55 or 65/16. These sizes give you more sidewall which also induces more sidewall flex and can affect handling characteristics.
Your new 18s will help a little because of being lighter and also reducing the size of the sidewall to achieve the same total diameter of the aforementioned 225/65/16 tires. Less sidewall means less sidewall flex and better handling characteristics.
Keep in mind that tire size is a balancing act. Shorter total diameter tires require less power to get rolling, but taller total diameter tires are better for highway eco. If you go too tall, your car will accelerate like crap and possibly negate any gains made on the highways depending on your commute.
Custom rims, wheel tire packages for your ride - RIMSnTIRES.com
Rims and tires is a good place to understand aspect ratios and tires sizes and can help you pick a larger tire.
HTH
__________________
-Mike
2007 Ford Focus ZX5 - 91k - SGII, pending upper and lower grill bocks - auto trans
1987 Monte Carlo SS - 5.3/4L80E swap - 13.67 @ 106
2007 Ford Focus Estate - 230k - 33mpg - Retired 4/2018
1995 Saturn SL2 - 256K miles - 44mpg - Retired 9/2014
Cost to Operate Spreadsheet for "The New Focus"