Firstly it depends if you are looking for a cardio workout stand up paddleboarding session or a sufing session. It will also depend on your skill level.
For your first time out you would be better off on very flat water on a day with minimal wind. Your body has got to get used to balancing on a relatively small board on the water so its easier if the water isn't moving you around too much! A local lake or pond is the best place to start out.
Alternatively if you can find a sheltered beach then use that. You want to keep away from river mouths to ensure you dont get swept away from the shore with the rip from the river. Also you need to keep an eye on the conditions - too much wind and your body acts like a sail and you get blown all over the place, and possibly out to sea. Anything over 10mph and you notice it. Over 20mph and it gets really hard and frustrating.
Keep an eye on the swell forecast too - for your first few times out you won't want to be in big crashing waves. Not only will you get fed up with swimming all day instead of stand up paddle surfing but you run the risk of damaging your board if a large wave breaks over you of sends you into the rocks etc.
The surfing website magicseaweed is a godsend for stand up paddleboarders - you can find a lot of surfspots listed on there with daily eyewitness surf reports. You will soon learn what wind and swell conditions suit your SUP skill level for your local spot.