If you don't know a lot about the market or dont have the time and energy to follow individual stocks, a pretty safe place is to just buy into an ETF like VOO or VTI. The are diversified and easy to follow. If at a later time you want to invest slowly in a individual company with a small amount, thats fine, but I would keep most of your money in an ETF, or a few different ones.
ETF (exchanged traded funds) are actually pretty intimidating to a new investor. Mutual funds are way easier to understand since they're not as complicated and certainly much easier and most everyone that does index investing uses mutual funds like VTSAX.
I personally see no reason to use ETFs ever for a basic index investor. There are advantages to them, but they're not a need for lazy portfolio investors. Why complicate things when there's a much easier/same benefits option?
OP - I'd suggest you read Jim Collins' stock series:
http://jlcollinsnh.com/stock-series/ (it is available as a book as well).
But it's pretty easy:
1. Get a basic understanding on how the market/investing works: read the series above, and check out the Bogleheads wiki.
https://www.bogleheads.org/wiki/Main_Page2. Figure out your IPS (investment policy statement). This is your blueprint on the why/how/when for your life/future.
https://www.bogleheads.org/wiki/Investment_policy_statement3. Figure out your asset allocation (AA) based off your IPS and your own comfort levels.
https://www.bogleheads.org/wiki/Asset_allocation4. Figure out what accounts you have to work with, and apply your AA to them as a whole. Here's a great list of "lazy" portfolios you can use to set things up.
https://www.bogleheads.org/wiki/Lazy_portfolios5. Once the IPS and investments are figured out, you can then also see that mortgage payoff fits with your future goals, your risk level comforts, etc... Conservative goals, then maybe paying off the mortgage is right for you. Realize you're comfortable with being more aggressive now that you understand how the market works, and you have stable jobs that mean you can pay the mortgage easily as it stands? Then that's your path...
And of course, ask questions here. There are no dumb questions - there are always folks here willing to help you understand how things work, whether this or that choice might be good or bad, how to do things with a mind for taxes... it may seem quite intimidating at first, but I promise if you can read things and ask questions when you hit a snag, you're going to figure this stuff out and it is really mind-blowing when it finally clicks for you.*
While I personally wouldn't pay off my mortgage unless it was crazy high interest rates, for you, that may be the right move. The debate over mortgage/invest is endless. But once you figure out the investing part and decide your IPS/AA and move forward on that, you can then decide (based off of your goals and future and such) whether to pay off the house, invest the whole nut, or a split down the middle. It all makes sense once you go through those steps...
*I am the poster child for going from OMG this stuff is too scary... to now being 100% confident in managing my own stuff. If I could do it, anyone can. Good luck!!