Hola Angie!
"I must study more" is:
"Debo estudiar m�s" or "Tengo que estudiar m�s"
and...
"It is necessary to study more" is:
"Es necesario estudiar m�s".
About what you wrote:
"Esta necesito estudiar m�s"
(May a make some corrections? ;-))
You have writen "Esta". Note that "Esta" is the translation for "This" (feminine), so, in this case, instead of using "esta", you could say:
"Yo necesito estudiar m�s"
Apart from that, the rest of the sentence is perfect!!
Good!