Attach yourself to your leash, safety first!! If you fall off and get separated from your board you could be in trouble!
Typically it is better to start off with an all-round SUP that's

easy to balance on, do your research.
Check the tides and wind direction, it can be very hard work when paddling against the winds and tides.
Practice your technique
Engage your core
Face the right direction and keep your head up, it can be very easy to lose balance and focus if you aren't looking where you are going.
If you aren't a confident strong swimmer, wear a life jacket.
Take a drink with you, paddle boarding can be very thirsty work!
Get yourself a dry bag and pack some essentials, such as - Spare change of clothes, phone (for emergencies, and photos), micro fiber towel, food (if you intend on being out for a long period of time) and sunscreen!!
If you intend on paddling in the colder months then remember your warm weather gear - wetsuit shoes/boot/socks, thermals, hat, gloves, scarfs.