Try using Rustoleum Restore.
The first step is cleaning it with both a deck prep cleaner and pressure washing (I would recommend at least a 2500 psi machine). You'll be amazed at what a difference just doing this will make! However, for long term UV and water protection a sealer needs to be applied. DO NOT USE A THOMPSON'S PRODUCT! Everybody has heard of them - they have the best marketing and the worst product, in my 22-year experience. I would highly suggest one of the many Cabot's products available at Lowe's. They have everything from a clear, natural finish which will simply draw the grain out, to a wide variety of color choices. I would avoid painting the deck at all costs. It will simply not hold up, plus gives you no UV protection. Since you've never treated the deck, I would also suggest starting out with as little color as possible. Once you apply color, you can never go backwards. So, start subtle and go from there. Most people just use a natural, semi-transparent the first go around and are quite satisfied. Good luck!
Be careful when pressure washing.. if the deck is old, you don't want to go hog wild or you will eat away at the wood. But it is necessary. After pressure washing, I would recommend giving it a light sand with an orbital floor sander (you can rent if you don't have) then using a tinted sealer (I like Thompson's - to each their own). I wouldn't bother using a stain because IMO, they all fade anyway. A tinted sealer allows you to add color without adding the additional step.
if u have a sprayer- use household bleach and water, spray and let the sun bleach.