Each story must be written in a consistent point of view. Here are the POVs that are most used: 1st person, 3rd person limited to one POV character per scene, 3rd person omniscient. In each case, the story can be written in present tense or past tense.
1st person present tense:
I study the options. Choice 2 is best.
1st person past tense:
I studied the options. Choice 2 was best.
3rd person limited POV present tense:
He studies the options and nods. Choice 2 is best.
3rd person limited POV past tense:
He studied the options and nodded., Choice 2 was best.
3rd person omniscient POV present tense:
He studies the options. He decides that choice 2 is best.
3rd person omniscient POV? past tense:
He studied the options. He decided that choice 2 was best.
If you write in first person, you are presenting the story as witnessed by the narrator, told to the reader almost in conversation. The first person narrator, though, does not usually acknowledge the reader's presence. (Example of acknowledging the reader's presence: I opened the diary. I know you think I shouldn't have, and you're right. But I did.)
An advantage of first person is that you can write in conversational style. The narrator can be less formal, taking on the expressions and mannerisms of the main character. A disadvantage is that everything the reader is told has to be seen by the main character. You can't write a scene that the character doesn't witness.
(James Patterson, and some other bestselling writers break this rule. They write in 1st person, but throw in some scenes in 3rd involving other characters, such as conversations between the villains. It's best not to do this until you have become a published author yourself. If you do intersperse 1st and 3rd person scenes in your first manuscript, you run the risk of an editor thinking you don't know how to write or accusing you of cheating by taking the easy way out.)
Writing in 3rd person can be done in two POV styles, limited or omniscient. Limited means being in one person's head in each scene. Usually, you write from the POV of the protagonist, the antagonist, or the character what has the most to lose in the scene. Everything that happens is seen by that character. Only the POV character's thoughts and feelings can be mentioned to the reader. The great advantage of doing this is reader engagement. The reader can become connected to the character, feel what the character feels, and come to understand the character deeply. If you want the reader to identify with and care about your protagonist, what better way then to have the reader experience what the protagonist is experiencing?
If you use an omniscient narrator, you are allowed to flip heads, to show what more than one character is thinking or feeling in the scene. Writing in omniscient might seem easier, but it's more difficult to do well. Often inexperienced writers try to write in omniscient, but end up bouncing around between limited and omniscient. Here's an example:
Harv watched the target overtop of his book. She hadn't realized she was being watched. He chuckled. A pro would have catalogued everyone in the vicinity.
But the target was aware. She was looking in a store window, and caught the reflection of the man on the bench, supposedly reading a paperback. Why did he never turn the page? She pondered her next move.
The above example starts in limited, Harv's POV, although it could be an omniscient narrator's account. Was Harv thinking about what a pro would have done or was that the narrator pointing out a fact to the reader? The comment of the target being aware had to be from the narrator, because Harv didn't know that fact. Then the writing flips to female target's POV, because she's asking herself why the guy never turned the page. The final sentence is from an omniscient narrator, who tells us what she was thinking. (We don't witness her thoughts.)
When writing in limited, stick with one character's POV for the whole scene. You can switch POV characters at the end of a chapter, or within a chapter if you leave an empty line or separator.
I suggest that beginning writers use 3rd person limited and try to get right inside the character's head. That's the best chance for engaging the reader and cuts down on the tendency to tell rather than show, a bad habit that tends to accompany omniscient narrators.
Regarding present or past tense: read my article on that choice.