Ummm...okay...let's see...

     So...bueno...em...every language has "filler words," or words that fill spaces while the speaker is thinking. In Spanish these are called muletas, or "crutches." If you're speaking and you don't want to break your flow, these words can come in handy. I've separated these words into categories for you below.

Okay
     To assent: ya, bien, bueno, está bien, claro (as in "of course"), vale (Spain), cheque (Honduras), pura vida (Costa Rica), órale (Mexico), de acuerdo, sí, okey (it's Spanglish, but in some places people do use it), ándale (Mexico)
     To start a sentence: bueno, a ver, entonces, ya, bien, pues, che (Argentina)
     Enough!: basta, ya, no más (these three are often used in combination--¡Basta ya!, ¡Ya no más!,  ¡Basta--no más!

There's a good list for "okay" in Span¡shD!ct, with some of the ones I list above and a few different ones.

Um...
     Este is the top word used for this, usually drawn out, like esteeeeeeee... and sometimes followed by an m (esteeeeeeemmmm). It just means, basically, "I'm thinking--don't interrupt."
     Bueno is sometimes used as a filler, just like "um," and does not necessarily mean "good." It just fills space.

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So...
    Then... (you're following a train of thought): entonces, o sea, o sea que, así que
     Therefore: por lo tanto, por lo cual, por consiguiente
     To an extent (I was so glad to be there that I forgot to eat): tan
     So what?: ¿Y? , ¿Y qué?
"So" is a multipurpose word in English, and I've only given you a few very common uses for it. If you'd like a more complete list of ways to use translate "so," there's an excellent list in Word Reference.

Hey
    To get someone's attention: ¡Oye!, ¡Eh!, ¡Ei!, ¡Oiga! (more formal, but still kind of rude)
     To object: ¡Oye!, ¡Pero oye!, ¡Hombre!
     To greet someone: ¡Hola!


Oops
     ¡Uy!, ¡Ay!, and sometimes ¡Uyuyuyuy! or ¡Ayayayay! A lot of times these expressions are accompanied by the shaking of a hand up and down with the backs of the fingers facing the speaker's audience. 


     Use these expressions correctly and you'll sound like you know what you're talking about. Even better, they will help you avoid using English, since the place where most second language learners fail to stick to the second language is usually with these sorts of expressions. If you stick to just Spanish in your communication, you'll be less likely to derail your train of thought. If you think of other expressions you'd like to know, ask in the comments and I'll be glad to help you out.