Well, such a ticklish question u gimme. U know Pinyin is just the ABC in chinese and the only way to type chinese characters into the computer. It's acceptable for u to write Pinyin to express ur thoughts just not so comfy for those chinese people. Same reason that could u know "Vremja den'gu daet, a na den'gi vremeni ne kupish" just is "Время деньгу дает, а на деньги времени не купишь" within a sec? Pinyin, sometimes, euqals that transliteration of characters just we barely get accustomed to. So if u know how to write, better to use characters rather than Pinyin. That means u have to remeber each of characters with its tone. And i don't know what "key characters" u mentioned really is. U mean radicals and components? If so, u'r right. they r just like the etymas in Russian, English etc. U could even guess how to read a character from the components while radicals, much indicate to the attributes or forms from the characters and thus, u could remember more components so that u could also know more different characters, sometimes these characters have the similiar or even same pronunciation. eg: 铜(copper, composed by 钅and 同) and 桐(tung, composed by 木 and 同) have the same component "同" whose Pinyin is [tóng]. And here both 铜 and 桐 have the pronunciation [tóng]. And remember many of characters lefted with radical 钅, they r related to metal materials, while characters lefted with radical 木, they'r much for trees. Now, an amateur as u'r, my suggestion, personally, learn Pinyin well for ur first step, it's really a must for anyone who wanna learn chinese.

For the adult learner of Chinese pinyin is essential. It is also the way to input Chinese into a computer, for native chinese speakers and everyone else, you simply type the pinyin and a list of characters comes up for you to choose from. Some input methods are smarter and faster than others, so find one that you like. You are right, often there it is very hard to find the meaning of a word, or sometimes even a sentence from pinyin, but without it it aside from having a Chinese reader with you at all times, there is no way to learn how to pronounce a new word. But once you know the proper pronunciation, including the correct tone, you wont really use pinyin when writing on paper or when reading. The key characters you are talking about are called radicals, and learning them from the very beginning will prove more and more useful the further you progress. Sometimes radicals point to a meaning, sometimes to a sound, and sometimes to nothing at all, so it isn't completely black and white, but still very important.

I am a university student, my major is English ,maybe we can exchange our language !

You can try to make some Chinese friends

If you just study, you'll learn these things step by step.