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.