Sometimes, just being able to identify them is helpful. They float or drift along, so just be aware and don't get caught in their tendrils. Others such as the beautiful lionfish, scorpionfish, and the much less attractive rockfish, have poisonous spines. Avoid them and they will avoid you. You may want to have a check-up from a doctor.
Some people have medical conditions that preclude diving.
Be honest with yourself about your physical condition. If you've been physically inactive, it would be advisable to start exercising on a regular basis. Even bicycle riding can make a big difference in your overall fitness. Many dive shops offer these courses designed to give you a taste of diving. Similar to resort courses, you go through some basic instruction and then go on a shallow dive with an instructor.