Yeah

"Yeah" is a more casual way of saying "yes". 

Say this to agree with something that a person said:

Yeah, no problem.

Yeah, I actually did it myself.

You can also say "yeah" when you're going to disagree but you want to make your disagreement sound a little softer.

Yeah, but then it'll be blocking one of the outlets.

"Yeah" sounds less formal than "yes." 

overall

You can use the word "overall" at the beginning or end of a sentence.

It's used when your general opinion about something is different from your opinion about some of the specific parts of it. For example:

Overall, I think we did a great job. There were some mistakes, of course, but nothing really major.

Here's another example. It means that you mostly like the camera:

The Canon T2i has some weak points, but overall it's a great camera.

Notice that "overall" can be at the beginning of a sentence, or after a word like "but". It can also come at the end of a sentence:

I'm really happy with how it turned out overall.

I'd say

This is a way to answer a question or give an opinion, when you're not really sure:

I'd say she'll probably be back at about 2:00.

"I'd" is short for "I would". So you can also say "I would say ___":

I would say that you can probably find one for under ten thousand dollars.

You can introduce the rest of the sentence with "that":

I'd say that I'm two or three weeks away from finishing.

"I'd say" can also go at the end of a sentence:

It takes about an hour and a half, I'd say.