Ako ay nasa zoo. = I am at the zoo.

Ako ay nagpunta sa zoo. = I went to the zoo.

“Katakot-takot ito” is not really wrong, but we rarely say it that way because, as Lyra has mentioned, we more commonly use “katakot-takot” colloquially to mean “an awful lot of”.  We say instead, “nakakatakot”, making use of the “nakaka” prefix, which can give a “causing (something)” meaning to the word it is attached to.  For example: nakakatawa = funny (bringing about laughter); nakakagutom = appetizing (causing hunger); nakakatakot = scary (effecting fear).

“Guhitan” is correct, but it will depend on how it is pronounced.  To mean that something has lines or striped, like the zebra, the stress is on “tan”.  If the accent is placed on “hi”, it will become an imperative, meaning “draw a line on” or “underline”. You may also say, “may mga guhit ito” (this has lines/stripes) to describe a zebra.

“Nagkaroon na ako masaya” translates to “I already have happy”.

Masaya ako = I am happy.

Masaya na ako = I am contended/I am already happy.

Pinasaya/Napasaya ako nito = This/It made me happy.  - focus is on the source of happiness

Nasayahan ako = (It) made me happy. -  focus is on the speaker

All of your other sentences are correct.

Ako ay nasa Zoo 

Ako ay nasa Zoo. Nakakita ako ng elepante. Malaki ito. Nakakita ako ng Kabayo. Mabilis ito. Nakakita ako ng buwaya. Katakot-takot ito. Nakakita ako ng zebra. Guhitan ito. Nagkaroon na ako masaya

Salamat Po!

My first attempt ever to write sentences in Tagalog. Any advice is appreicated

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Hello! The highlighted parts were the ones I'm going to correct. 

Here, I'm assuming what you want to say is "It's scary". The term Nakakatakot ito  would be much better. Katakot-takot is often used to say "many". 

For the second part, I think what you meant is that you had fun. Nagkaroon na ako masaya is a very direct translation and would mean differently. Saying, Nasiyahan ako, Natuwa ako or Nag-enjoy ako would be better. 

I hope this helped. :) 

Ako ay nasa Zoo

Ako ay nasa Zoo. Nakakita ako ng elepante. Malaki ito. Nakakita ako ng Kabayo. Mabilis ito. Nakakita ako ng buwaya. nakakatakot ito. Nakakita ako ng zebra. Guhitan ito. ako'y nasiyahan

Salamat Po!

My first attempt ever to write sentences in Tagalog. Any advice is appreicated