Of course, it's not all good. They can create a colony underneath your favorite plants and produce an unsightly mound. They'll eat plants and roots that are in their way. Some ants will corral aphids and actually protect them as they suck the sap from your plants; the ants feed on the excretion of the aphids, a "honeydew" sweet liquid that ants love. They can invade your home seeking food. Seeing ants in places you don't expect to see them can be a little unnerving, but they're not all bad; California has over 200 species of ants, and only considers less than a dozen as pests.
Dealing with ants is a good opportunity to practice Integrated Pest Management (see my blog,"Integrating Integrated Pest Management"). First determine if the ants really are a problem in your garden. If they're just walking around, checking out the area after waking up, then leaving them alone is easy. If you see a large mass of ants, don't panic, wait and see where they go and what they do; they may just be moving to a new home.
I rarely try to kill or reduce the number of ants in my yard. When I do, it's usually because large numbers have moved into a territory that I've identified for another purpose, like the middle of a vegetable bed. As long as it's not a harmful species like fire ants, disturbing the nest with a shovel may be enough for the little creatures to grab their eggs and move elsewhere. (Important note: if you really have fire ants, they should be reported to your county agriculture agent for identification and control).
If you begin to see actual plant damage due to ants or the aphids they're cowboying, it may be time for action. If they're in a location that is a problem, you probably want them gone. Take some time to determine where their nest is. This may mean following a line of ants, or even a single one, until it leads you to its home.
You may not need to kill a single ant to alleviate your problem. Ants seek out food and then return home by following a scent trail that they made. By disrupting the scent trail you can reduce the likelihood that the ants will return to the food source, like your kitchen. Soapy water will work; sponge off the area that the ant was walking to eliminate the trail. Outside you can spray it with a hose, but plain water is not completely effective, so using a bucket of soapy water is better.
For best control realize that you need to deal with the ants that you can't see. Spraying or dealing with the few on your plants or in your kitchen won't remove the problem. You need to choose a method that attacks them in the nest.
If you feel you need to eliminate the nest, you can use a few methods that are not completely toxic. Diatomaceous earth will act to desiccate the ants and kill them. It's commonly used to control slugs and snails so you just may have some in your garden shed. Sprinkle it around your garden beds and the ant nest.
You can make your own ant poison by mixing boric acid with sugar, honey, or jelly. Place it near the nest to avoid attracting other ants to the area you're trying to clear.
If you decide to use a commercial ant poison you should identify what kind of ants you have. Each region can have dozens of different species and some are easier to remove or kill than others. Ants generally look alike, but upon close examination you'll see that some are brown or black while others are yellow or red. You also want to be sure you don't actually have termites, which are often confused with ants. Do an online or library search to identify the species you're dealing with. The University of California, Davis has an online key here.
The reason for identification is that different ants react differently to the standard ant poison you can buy. While the poison may kill many of the ants in a garden, it won't kill all types and may actually cause some to move from the original location to other areas, like under or in your house.
Generally, ant baits work better at killing ants in the nest than pesticide sprays. The sprays will only affect the ants that come into direct contact with the pesticide while the baits will be carried into the nest by workers and transferred mouth to mouth and ultimately kill most of the ants.
The initial reaction by many people when they see ants is to want to kill them. I recommend that you stop and analyze their effect on your garden. Chances are they're having a beneficial effect. Don't assume that they're causing a problem. Even if there is a problem, don't rush to kill all of the ants. Ants are an important component in a garden's life force and eliminating them may disrupt the natural circle.
If you choose to take drastic action, practice smart pesticide practice. Deal with the ants directly and don't wantonly distribute harmful chemicals in your garden.
Be aware that there are ants throughout the world. The attractions that brought ants to your garden are still present and removing a nest just opens up space for different ants to move in. Once you begin trying to control ants, you may start a course of action that takes up a lot of your gardening time.