Good evening.,

In a week I have an exam for university.

Or “In a week I have a university entrance exam.”

I didn't prepare for ithem because I didn'wasn’t planning to go to university.

Or “…I didn’t plan to go to university.”

But I do it for my family.

Or “But I’m doing it for family.”

If I can't do it, my parents will be upset and be reminded about it every day.

Or “If I’m not accepted into university, my parents will be upset and remind me about it every day.”

I am trying to do something, but I don't think this helps.

Or “…but I don’t think it helps.”

Because people prepare for years...

In a week I have an exam for my university.

Good luck on your exam!

Just a stylistic choice: "In a week, I have a university exam" (where university is an adjective) may be a little simpler. Your sentence is easy to understand, though. It just needed some extra articles.

I didn't prepare for ithem because I didn't planning to go to university.

If there's just one exam, we're going to say "I didn't prepare for it" rather than "I didn't prepare for them"

The other half of this sentence is difficult in English! Compare:

"I wasn't planning to go to university" (past continuous tense, which requires an -ing on the participle "planning")
vs.
"I didn't plan to go to university" (simple past tense in the negative, which does not use -ing on the participle "plan")

If I can't do it, my parents will be upset and be remind me about it every day.

"If [present] then [future]" structure. This is a good one to practice.

I am trying to do something, but I don't think thisit will help.

Remember to use "to" with your infinitive verbs:

I am trying to do something

Because people prepare for years...