Congratulations on your new boat! If your boat was just launched the yard will generally not put more tension on the shrouds and stays than is needed to keep the mast upright at the dock. If you can find some similar boats in your marina or nearby, give those shrouds a wiggle or measure their tension with a Loos gauge to get an idea what other rigs feel like. Racing boats are generally set up more tightly, and most have a way to adjust backstay tension underway (tighter for upwind and windy days). Once you get the mast straight side-to-side, and is raked the amount you want, setting the tension to twenty per cent of breaking strength is one place to start for 1x19 stainless steel wire. Also, once you get the rig set the way you want at the dock, go sailing and see if the shrouds on the leeward side go slack when the rig is loaded up. More than a very small amount is slack is bad. Also check to see if the mast is bending side to side; sometimes the top will bend off to leeward or the middle will. After the initial tensioning, the boat will bend a little with overnight and the next day and some re-tensioning of the shrouds will be needed after a couple days (the boat is plastic with fiberglass fibers, after all, not metal). Usually the second adjustment will do it.
I recommend putting some lubricant on the turnbuckle threads before turning them. I use a small dab of anhydrous lanolin at each thread, but there many lubricants. Also, it is better to use a wrench on the body of the turnbuckle and a second wrench on the pin, rather than a wrench on the pin and a screwdriver through the turnbuckle, although the screwdriver is a common practice.
I suggest you write down your initial and final settings so you can come back to them later. I also record the number of turns so I can see later how many turns gave me the last increment of tension.
Some people like stainless cotter pins and some like rings to lock the turnbuckles. I use bronze cotter pins. They are much easier to bend and unbend, but they are plenty strong enough to stay in place.
When you're done, covering the turnbuckles with rigging tape or a cover will keep sails and ankles from snagging on the cotter pins/rings.
If you're new at this it would be worth a couple hours of a professional rigger's time to set it up for you the first time, and to look around to see what else could be adjusted for better sailing and security.
Good luck and have fun.