What steps should American voters take to be well informed first before forming an opinion whether America should get involved or not with foreign affairs?

Be well informed. Read newspapers, watch news programs. Be up-to-date on foreign affairs.
(Not that voters have anything to do with what our leaders decide.)

What @kritiper said plus check past votes (ie Pence), talk to military who can give you perspective.

Well, they’re sure as hell not going to get anything close to objective reality through all the regular channels like the media. For the average American it’s an unattainable goal unless they have the money and freedom to travel and see the reality of the rest of the world. A few may be intelligent enough to figure it out on their own by digging in and researching properly if it’s even possible for a given situation. That pretty much leaves all of us out of the exclusive club of people in the know. I honestly don’t know of a good and accessible way to do this for the average person.

They should research their elected officials before buying into the choices given to them by the media. Beyond that, John Q. Public should have no input into foreign affairs. That would be way too cumbersome.
As for how to research the candidates, I suggest visiting each of their websites and seeing what they stand for. Take a look at what they say they want to do and run it through the mental filter of reality. Consider if their goals are realistic and what the impact to the country would be.
Take the idea of Medicare for All as an example. Lots of enthusiasm as the candidates all say it is a great idea. But the financial cost of such a venture is estimated to be $32T over 10 years. That averages out to be $3.2T per year. Our GDP in this country is only around $3.6T/yr. So 89% of all money the government takes in would have to pay for this one program. That seems unrealistic and it would mean that other services would have to be cut drastically OR taxes would have to go up significantly across the board. Significantly like 100% higher than they are.
https://www.thebalance.com/current-u-s-federal-government-tax-revenue-3305762

https://www.forbes.com/sites/sallypipes/2018/07/09/choking-on-the-cost-of-medicare-for-all/#428185b56f3c
And I think people need to realize that there is no program or goal that will make everyone happy. So I, personally, like to look at the candidates that aren’t afraid to tell me bad news. They seem more honest to me.
Another litmus test I have used is one that can only be used on incumbents. Write them an email…ask them a question. I have found 3 reactions to this. The first is that I get nothing back in return. The second is that I get a form letter back, thanking me for my interest…but nothing else. The third generally starts like the second…a form letter…but is shortly followed up by an actual letter back from that representative’s office addressing my specific letter. That third one tells me that person is still engaged in how his/her constituents are feeling. These people I tend to favor at the ballot box as well, though it their values and goals are drastically off from mine that impacts as well.

The more TV channels you watch, magazines, newspapers, etc. that you read, the more objective your informational reality becomes. It may not be perfect, but it’s the best one can do to be informed outside of putting oneself in a direct line of fire.