Masu vs Shimas

Just a beginner here. I'm hung up on the difference with verb form adding masu vs verb form
adding shimasu. I plucked these examples out of the lessons. I'm trying to figure out how they would
differently translate into english. Is one a future tense. Many thanks in advance?

おさかな　を　たべます。 I'll eat fish.

べんきょう　します。 I will study.
 

おさかな を たべます。
お魚を食べます。
The verb is たべます、which is the polite form of たべる ("to eat"). It can be translated as "I eat fish" or "I will eat fish". Usually context will make it clear whether the verb is present or future tense.

べんきょう します。/ 勉強します。
This is a NOUN + する construction.
This page describes NOUN+する in more detail (scroll down to point #7):
https://www.thoughtco.com/extended-use-of-suru-2027858

べんきょう  (勉強, "study") = noun, and it's used together with the verb する ( "to do") conjugated to its polite form, します。Whether it is present or future tense depends on the context. For example, if the sentence is 「明日は勉強します / あしたはべんきょうします」 the word あした (明日, "tomorrow") makes it clear that the verb is future tense. Tomorrow I will study.

Japanese has different types of verbs. Like "ru" and "u" ending verbs. There is also the "suru" which can be tacked onto the end of a word to make it a verb and conjugate.

When you look at the verb eat. The verb is taberu so it becomes tabemasu in polite form.

When you look as the verb study it is benkyou-suru. It is a "suru" verb so will become benkyou shimasu.

It does get more complicated from there as not all Japanese will use the polite form and will just use plain form in common language. It is best to learn all forms.