PM me and I'll be glad to answer whatever questions you might have. Because it's deer season, I wont' be at the match. Although it sounds like eshell will be there though. So I recommend you PM him. He's a great guy and will do his best to work with you and teach you all he knows.
Let me tell you what comes to mind right off the top of my head....
You've got a good rifle, with good optics (14x is enough to shoot 1k. I've shot 1k with a fixed 10x). The only issue might be your scope set up. You will need about 38moa 'up' to be able to shoot 1000 yards. You can determine that by zeroing your rifle at 100 yards, then turning your elevation turret and count how many minutes 'up' you have. If you don't have enough elevation, you will want to get a 30moa canted base or use the Burris Signature Rings with the offset inserts. In the alternative, if you have a mil-dot scope, you will be able to use a combination of turret adjustment and the dots. I know that's a lot to digest, PM me with any questions.
Shooting a 308, 175smk with RL15, that's a good load (I've shot 175SMK with 44grains of RL15 and my current load is a 175VLD with 44.5 RL15). If you happen to know your muzzle velocity, that will be helpful to those trying to get you on the target at 800 yards. Don't worry if you don't know. Anybody there will be able to get you on the target.
If all you've got is 300 yards to practice, then use it. Put up small dots and try to hit them. Concentrate on the fundamentals of shooting.....breath control, trigger control, making sure you have the same cheek weld every time.
You can find directions to the range at www.quanticoshootingclub.com
Generally speaking, registration will begin at 0830 and shooting will generally start around 0900. Cost is $35 payable in cash at registration. There are no forms to complete. Because you're 15, you might need a parent/guardian with you. PM ehsell with that question - he will probably know.
Here's a thread where eshell provides a good list of gear to bring to the match. The only thing I will add to the list is to dress appropriately and generally speaking, overdress. You can always take a layer off if you get too warm. I've been cold on that range and nothing makes you miserable like being cold. Also, if you even think it's going to rain, bring rain gear. I shot a great match in the pouring down rain. Quantico 9th Modified F-Class match October 4th - Maryland Shooters
That's a good start. PM me if you have any more questions.