“I was thinking a shark or something along those lines.”

You're considering getting a tattoo. You're not sure exactly what you want to get, but you have some ideas. A friend asks you what tattoo you want, and you say:

I was thinking a shark or something along those lines.

I'm thinking (something)

Talk about a decision with this phrase:

A: Which one are you going to get?

B: I'm thinking the blue one.

You say this when you're not completely sure of your decision yet.

something along those lines

This phrase means "something like that".

I think you'd be good as a fitness instructor or something along those lines.

He said "Never let anyone tell you what you can and can't do," or something along those lines.

This phrase is just a little more intellectual-sounding than "something like that".