San Francisco ? Is Twitter the next Google, the next Pets.com, or something in between? It may have begun answering that question Tuesday, with its long-awaited first step into advertising.

The startup is trying to make money without alienating the tens of millions of people who have gotten used to tweeting and following friends, celebrities and others without commercial interruptions. Just as it has through most of its four-year existence, Twitter is treading cautiously.

The new ads, called “promoted tweets,” will pop up only on searches at Twitter’s Web site, and the messages will be limited to a small group of test marketers including Virgin America, Best Buy Co., Sony Pictures and Starbucks Corp. Fewer than 10 percent of Twitter’s users were expected to see the ads Tuesday, but the messages should start appearing on all relevant searches within the next few days.

The move heralds a turning point for Twitter, which has held off on selling ads even as its widening audience turned it into an obvious marketing magnet and investors poured $155 million into the San Francisco company.