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: 
First off, with the preposition *by*, we would normally mention a specific time or point in time. I will submit my assignments by 10 tonight or by midnight tonight. Or you can say *by the end of the day*, because that also refers to a specific point in time. (*By tonight* does not really specify a point in time, unless there is a previously defined specific time that *tonight* refers to, such as a deadline that everyone knows about.)

As far as which construction to use, either one is fine. The first one has more of a promise meaning. The second one has a more matter of fact meaning. I explain the difference in detail [here](https://ell.stackexchange.com/a/48043/6951)
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.