The best bait for first time fishing is ordinary earthworms. Earthworms as bait can catch the widest array of fish species out there. If you aren't sure what the easiest fish to catch is where you are fishing, earthworms will have you covered. Some other benefits of earthworms are that they are cheap and readily available. You can dig for them in your yard (or possibly even near your fishing spot). They are also a common bait found in bait fridges at most stores where fishing tackle is sold if you would like to purchase worms instead.
If you think worms are gross or you don't like fishing with a live creature on your hook you still have options. Berkeley Powerbait and Gulp products will catch the same variety of species as regular earthworms. There are a ton of options to choose from but angle worms or honey worms are two great options.
What about saltwater? Earthworms can also be used in saltwater but they will die a lot faster. Dead earthworms catch fish but most anglers agree that live ones give an extra advantage. You can instead dig up marine worms like bloodworms, tapeworms, ragworms, or lugworms but they are harder to find and can be expensive to purchase. Also bloodworms can bite you, which isn't a huge problem but for the complete beginner a bait that can't bite back is probably better option. Fishbites Bag O' Worms Bloodworm is great alternative that can catch anything a real bloodworm will but is cheaper and easier to use.
Yet another option is using grocery store baits like bread, corn, or marshmellows. The caveat to these baits is that they won't catch the same wide variety as worms will. If you are sure your target species eats one of these baits, you might be able to load up on bait right from your kitchen.
Fishing Advice for Beginners