Question: Which one is appropiate?

> 
> I will submit my assignments by tonight
> 
> 
> I will be submitting my assignments by tonight.
> 
> 
> 

I want to express that before the end of the day, I will submit my assignments.

Answer: 
I will submit my assignment by tonight.

I will be submitting my assignments by tonight.

Both the sentences express the future and are interchangeable, with a slight difference in meaning.

The first sentence formed with will + bare ininitive has the overtures of certainty, and intention and emphasizes the action. On the other hand, the second sentence formed with will be + present participle is merely a statement, which means that the assignments will be submitted in the normal course of events or a routine. It's therefore considered to be usual or not so promising as the first sentence.
In this case, "I will submit" is the correct usage.

"I will submit" focuses on the fact that there will be an event, where you submit the assignment. The listener can reasonably assume it will be swift.

"I will be submitting" focuses on the act of you engaging in submission. There will be a submission, but it concentrates on the time period where the submission is occurring, rather than the fact that it happens. I would use the phrasing "I will be submitting my assignments by tonight" if the listener knows I intend to submit my assignment in the form of an interpretive dance in the cafeteria drawn out over 3 acts, and I will be intentionally disrupting normal class to do so.

This is more obvious in other verbs. "I will pray for you" may be as little as "I'll pray for you once, but then I'm going about the rest of my business." "I will be praying for you implies a longer duration and many prayers. (*though some people will say "I will pray for you" and pray many times over a long duration. Welcome to English. We're not as consistent as we'd like.*)