Tax Laws?

I'm starting this lovely Memorial Day by researching/begging for help with understanding tax laws. Apparently in Nebraska, you have to pay taxes monthly. I had my very first official paying client yesterday and it's got me hot and bothered about tax laws. (obviously by "first official paying client" I mean "first client who paid me in a check - with a paper trail - and not with equipment/coffee/museum admission/a fist full of cash")

I needed an EIN back in April to open some accounts with professional photography services. I didn't really think much of it until this morning when I made an invoice for my client and I got all hung up about whether or not I needed to include sales tax in it. I'm thinking the answer is probably yes. But maybe not?