First time choosing a bookie. What matters most?

Hello everyone. I am completely new to the world of sports betting. I want to find a bookmaker to place a few small bets on weekend tennis matches. However, I feel overwhelmed by the number of betting brands online. It seems like every company claims to be the best. I want to know how an absolute beginner can separate good platforms from bad ones. What specific criteria should I look for during my search? Are big registration bonuses a sign of a high-quality website, or should I avoid them? I really need some objective parameters to compare different sportsbooks. I want to be sure that the platform is legal and will pay out my winnings without any sudden problems. Any recommendations or general tips on this topic will be highly appreciated.