A DNA ladder is a set of DNA fragments of known size. Run it in one lane of your agarose gel and compare your sample bands against it to estimate fragment size and approximate amount.
Pick a ladder whose range covers the fragments you expect. For small PCR products, a 50 bp ladder (50 to 1,500 bp, 17 bands) gives fine resolution. For typical PCR products, a 100 bp ladder (100 to 1,500 bp, 11 bands) works well. For plasmids and large fragments, a 1 kb ladder (250 to 10,000 bp, 13 bands) is the better choice.
Most ladders have one or two brighter reference bands. On the Helix 1kb ladder these are the 1K and 3K bands; on the 100 bp ladder they are the 500 and 1,500 bp bands. Find them first, then count the bands above and below.
Load the recommended volume, usually 5 µl per well, and use the same gel percentage and buffer for ladder and samples so they migrate under the same conditions.
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