Level 18 myth been in ranked pvp since I started and I keep losing!. the highest points I got was 510. so basicly my points went up and down. like 510- 360 then I made my way up to 495 then kept losing. HELP
First, pay attention to how the other players beat you, and see if any of their strategies might be things you also could try. And also, pay attention to what you have done that has had some success. For me, this simple practice made the most dramatic difference in my PVP play.
I stopped looking at the other wizards as my "opponents" and started looking at them as my "teachers." They regularly teach me effective strategies, both when they beat me (revealing my weaknesses and showing me other good styles of play), and when I beat them (showing me tactics that are effective for me to use against particular kinds of opponents).
Second, try looking up on the web some of the Duelist websites related to Wizard101. There are some very detailed, school specific lessons out there that cover everything from gear to pets to spell deck selection to effective spell combinations and mastery amulet choices for effective out-of-school secondary spell tactics. (For myself though, this can get to be too much -- there is so much detail in some of these sites that I go on overload.)
At your level of myth, in 1v1, minions can help a lot. Minions are a school specialty that are especially irritating to opponents at lower levels (at high levels, though, they become liabilities because they are trap stealing strategic morons). Also, the timely and judicious use of TC in your own school (quake, medusa, and basilisk especially) and in other schools (surprise!) can be very helpful.
Thirdly, for PVP, if you don't have good gear and a good pet that heals, first of all, and then also does stuff like critical-boosts, pierces, blades, etc., then you are fighting an uphill battle. You need these things in PVP.
Finally, just keep at it. It takes a while to get a style of play that you like.
Finn summarized just about everything you need to know. I would definitely take a look at some of the other 'sites and forums dedicated to Wizard101 PvP. Gear aside, training point allocation and deck construction is the most important aspect of successful dueling. I also can't emphasize what Finn said about paying attention to your losses enough. You need to know their school as well as your own; know what they're capable of casting at 'x' pips and adjust accordingly. Most importantly, don't give up!