I assume you don't know how to sight read at all. May I ask how you have been learning your pieces? I'm not very good at sight-reading myself; however I don't know many other ways to learn pieces. Synthesia and playing "by ear" are really the only ways I can think of, and the latter is not a very reliable method. I am a fan of Synthesia however, and that can be used as temporary alternative (NOT permanent, for various obvious reasons).
I don't know any good websites or videos, so I'll just sum it up myself.
The act of sight-reading is mostly dependent on your ability to read while playing; that is, how well you can read ahead, know what to play and play at the same time.
There are two major parts of sheet music.
Now, the most important part of sheet music is obviously the notes. You'll have to learn different abbreviations for the notes (GBDFA - Good Birds Don't Fly Away, etc.) really quite generic advice, but extremely effective for beginners.
The other thing is the theory behind the scores. You MUST have basic theoretical knowledge to have any hope at sight-reading. Things such as Dal Segno al Coda/Fine, ritenuto/ritardando, pauses, glissandos, trills, grace notes, repeats, pedaling (IMPORTANT), etc. If you do not know basic theoretical concepts, LEARN YOUR THEORY. I cannot think of more important advice than this.
This may be unreliable, especially coming from someone who doesn't sight read for anything besides learning a piece, but I can tell you that this is what I have been doing for about 10 years now.
ALSO: do not only rely on sheet music to play a piece. Listening to a recording is just as effective; possibly giving new insight into how to play the piece. Another piece of advice is to practice sight-reading; things like beginners pieces, to 8th grade phrases are effective in enforcing sight-reading ability. Also, there are many books dedicated to teaching sight-reading, and I promise those will help a lot. Practice a lot of right hand only, left hand only, different rhythms, start pieces at slow tempos, gradually get faster; these are very important and effective practicing methods.
I hope this helped, and hopefully someone else comes along with a in-depth website to help train you.
Topic: Need Advice with Sheet Music