When in doubt, vote Republican. Unless you have some local race where you have personal knowledge of some exception to the rule.
The exception, by the way, has to be a so-called "third party". I believe that one cannot vote for a Democrat for any office without committing MORTAL SIN. A vote for a Democrat is always a vote for Satan.
On the other hand, voting for a Republican can also be a bad vote. It may be that the Republican is the "lesser of two evils", and if it is, one could morally vote for the Republican. However, one would not be obligated to do so. As silly as this sounds, a good place to start is the candidate's own website.
Personally, I never vote in an uncontested election. We have some local offices (i.e., county sheriff, recorder, surveyor, and a couple of others) in which there is only one person running. If that person doesn't vote for himself or herself, she doesn't deserve to be in that office.
In national offices, I will tend to vote for the Republican. For State and local offices, I pay attention to what's going on in the news and make decisions based on what I know is happening. If I don't really know anything about the candidates (for example, the local school boards) I don't vote in that particular election.
If your State has already had it's primary elections, the November, the national office candidates are already set. President Trump has already made his recommendation to you: Vote Republican or else the Democrats will try to destroy the country--and he's right.