The line you purchase needs to be designed to work with the trimmer or the machine will not work as efficiently or at all. Worse, it could damage the trimmer. So if the label on the old strings and lines are not readable, you need to refer to the owner's manual. Each model has a range of styles and sizes of strings or lines. Quick reminder: If you've had to replace the trimmer head, make sure you purchase replacement strings or lines for the new one.
If you purchased a fixed-head trimmer, you'll need to buy pre-cut strings. They're quick to change, but you will be changing them more often. If your grass trimmer has a bump or twist feed, you'll need a spooled line. Changing the line is a tad more difficult, but you don't have to do it as often.
Whether you purchase strings or spools, make sure you purchase the right diameter of string or line. The bigger the diameter, the more the strings and lines will be able to cut through taller and thicker grass. They'll also be more durable, not breaking every few minutes. The small line diameter should be used for cutting grass. The middle-sized is suited for grass and weeds. The large diameter strings and lines are for grass and all small and large weeds.
In addition to the diameter of the line, consider the shape. It makes a difference in the cut. Round is the most common. Without any edges, though, it tears more than cuts the grass. A twisted line has more edges and cuts better than a round. But one with more sides and sharper edges will give you the cleanest cut. Edge shapes include six-sided, eight-sided, serrated and more.
In addition to keeping replacement strings and lines on hand, consider purchasing an extra battery. Charge both. Install one and start trimming grass. If the battery runs out while you're working, slip out the discharged one and put it on the charger. Then quickly plug in the other one that's been charging so you can get back to working in the yard.
Replacement parts for grass trimmers are easy to find online. And having extra strings or line and batteries on hand will keep you moving even when your trimmer runs into a snag. Sears can help you stock up to make your lawn care easier.