Getting nervous for first ocean swim-- any tips?

Take your wetsuit to the pool and swim some laps with it on to see how it feels, they feel a little funny at first.  You'll feel some slight pressure on your shoulders but it just takes a few yards to get used to it.

 Also, don't worry about the ocean swim, once you get out past the waves there isn't much different except you have to keep your sight.  About every 6-10 strokes, lift your head up slightly and make sure you're aimed just outside the buoy.  If you're not, fix your angle and keep going.  My biggest mistake on my first ocean swim was not sighting correctly and I ended up zig-zagging and swimming much further than I needed to.

 Tighten your goggles because the waves may knock them around if they're too loose, but not too tight that they're digging into your eye sockets.

 Most of all, just have fun, ocean swims are my favorite part of the sport.

Which ocean?

I don't want to scare you, but I have had some ocean swims that were kinda gnarly. If there is rough surf your entry and exit can be tricky if you've never been in the ocean before. When you are swimming toward shore you want to look behind you during the breathing cycle to see if a wave is overtakiing you. If you don't see it you can end up inhaling water as it goes over your head. Rough breakers can also make you tumble.

Also at some beaches the surface bobs enough that you can't see the bouys all the time. Its good if you have land marks like trees or telephone poles to keep you oriented if you can't see the bouys or other swimmers.

But these are in rougher conditions. usually once past the waves its pretty smooth.

Providence HIM was the first time I was in the ocean in years. I read there would be no waves but there were some. Not major but still. 

Anyway, I made sure I got some practice swimming in before my wave just to get the feel for it. The biggest thing I found was how bouyant I was in salt water with my wetsuit. Amazing! Oh, and it was my best swim so far. *shrug*

You'll do great!!

Edit: Oh, and if you can watch other waves enter and exit.

If I find myself panicking, which has happened in the past, I start counting strokes like I do in the pool.  I usually count 18, similar to a length in the pool, sight on 18, and then count starting over.

This will calm your mind down and get you settled in.

 Have fun with it.