New to pvp? Don't worry, lots of us are.
Losing a pvp fight is pretty embarrassing, especially when you lose every time you pvp.
I used to be like that, but after months and months of practice on my favourite pvp server, now I can pvp against some of the best pvpers on the server and (sometimes) win. :P
Here's a list of things that helped me get better at pvp and defeat my enemies.

1. If you really, really suck at pvp, become a potionaholic!
Many pvpers say that using a ton of potions in pvp is for noobs. Well, if you're a noob, do just that! Your potions will make up for your lack of pvp skill. Make a ton of different potions to bring into pvp. They give you a huge advantage over someone who's better at pvp than you.
Usually when I pvp, I bring at the minimum an extended strength potion, regeneration potion, and several healing potions. If you are pvping someone with a fire aspect sword, don't forget your fire resistance potion. If you are pvping multiple people, bring on the poison pots!
You may be thinking, that's a ridiculously large amount of potions. Try hoarding potion ingredients in a secret chest and make huge batches of potions at a time.
And what if someone finds out you're a potionaholic? Well, hopefully they won't ever kill you and find your huge pile of glass bottles in your inventory.

2. You'll need good equipment.
I used to be an armour hoarder. I would stash dozens of sets of really nice armour in my chests, but never use them. Then one day my friend convinced me to use a set, and I won the pvp fight. It was awesome. I then used up all my armour and became very poor. :P
It's a fact: good armour helps you win! You take less damage and although you have more to lose, you have a lower chance of losing.
Likewise, you'll want a good sword. A sharpness IV sword combined with strength II potion can kill someone with prot IV armour in 5 - 7 hits. Fire aspect swords are great if you know the person you are fighting doesn't have fire res. armour or potions. Take the time to get good equipment, and you'll become much better.

3. An apple a day keeps the doctor away.
Stock up on golden apples. These are made by taking an apple and putting gold nuggets around it. Golden apples help your health regenerate. Eat a gold apple whenever your health is getting low. Some pvpers use mushroom soup, but it's disabled on many servers.
You may also want to use god apples (notch apples) when fighting someone really good. However, don't overuse them or you'll get a terrible reputation on the server. As one person on my server said, "An apple a day keeps the doctor away but not freaking 8!".

4. Mcmmo skills count.
More and more servers are getting rid of mcmmo, but on servers that have it, take advantage of it. Pick your specialty weapon. Some people do archery, some do axes, some do unarmed, and some do swords. Personally, I prefer swords. However, each weapon has it's advantages.
- Archery is extremely useful when keeping someone away from you or if someone has a knockback sword, or when it's long distance.
- Axes are very powerful but can't be enchanted as much as a sword.
- Unarmed is useful if you get disarmed.
- Swords is multi-purpose and most commonly used.
Try training your skills in the mob arena if your server has one.

5. Learn to click fast.
When pvping, you have to click really, really fast. Try alternating between left and right clicking (right clicks block if you are using a sword). Only practice can make you better at this. Spam-hitting is a technique used by many pvpers to keep their opponent from getting too close.

6. Dress the part.
Choose appropriate enchantments for who you are fighting.
- Projectile protection if your opponent is a good archer.
- Fire resistance if your opponent has a fire aspect sword.
- Protection for anything else. No better than prot IV though, that's a waste.
Likewise, do the same for your sword or bow:
- Don't fight an archer with a knockback sword.
- Fire aspect sword is good for unenchanted armour people.
- Sharpness IV swords is good for prot IV people. It does a lot of damage.

7. Make yourself seem like a worse pvper than you are.
For a while, I ran around my server saying, "I SUCK AT PVP!".
Then someone in dia armour told me to jump in and kill him. I did. He was shocked.
The point is, if you say you are a great pvper, people will prepare to take on someone better than themselves. Then you lose. However, if you say you're not great, people won't be ready for your sudden burst of awesomeness. This trick won't work long as people will soon realize that you're not that bad.

8. Be a good sport.
If someone kills you, don't call them a hacker. Sure, they might have hacked, but you'll just become annoying. Try to say "Good fight!" after a fight no matter if you win or lose. And never tp out. It's a great way to show you're a coward. If you're going to die, you might as well go down fighting.

My tips aren't great, but I hope they helped total pvp noobs. :)
If you need a good pvp server to play on, try pvp.minevival.net. Just don't ask to pvp me, I won't. :P