Well its one part gear issue and most likely another part you being new to the priest class.
I will hit on some gear points first then after rotation/mod tips.
First a shadow priest raiding spec.
http://www.wowhead.com/?talent#bVcbhZZGxfVRfzcfqfzAo
This is one of the main shadow priest raiding specs. there are a few out there that differ slightly. but this is probably the most cookie cutter spec for a shadow priest.
also using the above spec you pick up full hit tallents. that means your hit cap is 288.8 hit needed.. just go for 290 to make it easy. your new.. min maxing is not your thing yet.
at the moment getting hit cap is your very first priority. once you get hitcap you will want spellpower. then crit.. obviously this is a very basic explanation i will not go into the whole crit/haste issue or anything like that.. just remember come 3.3 haste will be insanely valuable to you even double the value of crit.
so ya.. those are the two points you should go for asap. spec and hit rating. very basic as you are very new.
lastly is your casting rotation.
On a raid boss fight your rotation should be Vamperic touch, mind blast, devouring plauge, mind flay (till 5 stack shadow weaving) then shadow word pain..
after the initial rotation you go into your dot upkeep cycle while keeping mind blast on cooldown and filling with mind flay. for you being a beginner just focus on that rotation.. try to not clip the final ticks of your dots.. and always clip you mindflays at 2 or 3 ticks.. never 1 and never inbetween.
a good mod for this is quartz casting bar with the mf2 addon. also you can use quartz to keep track of your dots in a very precise manner.. to make them easy to manage.
playing a shadow priest is like trying to put a puzzle together.. the tighter your puzzle pieces (rotation) is.. the better you do. your goal is to make everything fit together perfectly with minimal dead space.
But i hope you realize that the shadow priest is not an easy class to "top charts on". it takes insane amounts of time skill and gear to become so efficient at the priest class that you can compete with every other class.. its not easy and its not a given right like most classes. this is not a one button win spec. i have played every class in the game and i must say that after the affliction changes shadow priests are the most complex dps spec in the game..
hope this helps a bit.. enjoy the shadow priest..
also go to shadowpriest.com for some great shadow priest guides