Though I'm over 50, I still recall casting my first vote. Congratulations.
You're to be commended for taking the vote seriously and for voting issues, not parties or personalities.
Recall civics classes, too. Candidates will do or say a lot to be elected that they either know they can't accomplish, or learn they can't accomplish. Look at their plans for getting their way. Note whether they have records of compromise and bipartisanship. Note whether they.have records that are consistent with their current words.
For incumbents or those who have been in Congress, look at
www.thomas.gov at legislation they've sponsored. What issues have they cared enough about to sponsor? What have they cosponsored? What have they worked hard enough at that they have cosponsors and support from both sides of the aisle? What have they voted FOR or AGAINST simply because it was expedient, but didn't mesh with stated words.
You can spend a lifetime doing this, and some do. Pick an issue or two or three to start. Look at whether the candidate's plans are realistic, not just making promises.
And have FUN!