i'm beginner at game development and i started to design some levels and environments but should i download my assets or learn how to create them like 3D modeling ,texturing etc if i want to have a job in the industry as a level designer ?

Generally a level designer starts with a "white box" or "gray box" world, where barriers and stuff are filled with simple placeholders. 

3D modeling and texturing are art roles, necessary skills if you are planning on being a game artist.

Good level design is hard, and does not require the fancy models or textures to build. Simple boxes and primitives should be enough to begin navigating and evaluating the level even though it may not have the full visual feel.  Good level design is difficult. In single player levels, building worlds that provide an amazing growth curve and implicit tutorial. In multi-player games building worlds that provide complex environments that are fair for all sides, providing different yet fair opportunities for everyone, that can be learned quickly and easily yet are always off-balance so advantages can be had but can also be readily overcome.

As you are looking for an entry into the game field, level designer is an extremely rare job. A studio may run for several projects in a row without hiring any. Higher level designers can often design the worlds to fit their mechanics and pass it over to artists to flesh out.  Art roles are extremely common jobs, usually a few are hired every project both as the studios grow and as people naturally change jobs.

31 minutes ago, frob said:

Generally a level designer starts with a "white box" or "gray box" world, where barriers and stuff are filled with simple placeholders. 

This is also known as "blocking". Recently a lot of developers have shared images of their raw levels, you can find most them on #blocktober.

5 hours ago, kane david said:

i want to have a job in the industry as a level designer ?

You need to know how to implement stuff.

Your job will be designing corridors so that characters and AI don't get stuck, designing levels that play fairly on both sides or guides the player in single player. Designing things like bridges that break and so forth.

In other words 3D modelers and environment artist produce the assets that you want, so that you can make the level.

The most important things to learn for a level designer is lighting, staging and most importantly communication. The level designer is the link between artist and programmers.

Most of the level designers I worked with did know some 3D modeling. Not to a point where they could make all the stuff they needed but to a point where they could make small adjustments to the art we provided.

Level designer often know a lot of code, so that they can create intractable pieces.

On 11/24/2017 at 5:11 PM, frob said:

level designer is an extremely rare job

so who could do this part if it's hard i mean how they mange levels without level designer ?

To be fair, I did say GOOD level design is hard. 

Creating levels that are balanced, or better, "perfectly imbalanced", is extremely challenging even for experienced veterans.  Such levels are design that every player and every set of options has different yet approximately equal advantages available to them, and whatever advantage a player can take, another player has an option available that overcomes that advantage.

Often that means each character type includes both a strength and a weakness, the strength overcomes other character types and the weakness is overcome by others.  With sufficient skill each player can defend against their weakness, but doing so tends to mean focusing on the weakness so they gain no use of their strength.  Thus each option has similar levels of power and is balanced, but each option can overthrow others so they are imbalanced.

Other times that means intentionally limiting resources, such as having six zones but only five characters, or having three zones with five characters.  No mater what choice they make the game will be unbalanced, yet it is balanced enough that the game can be played for an extended time.  Coupled with a perfect imbalance of character types, the game can be replayed for years if popular, as LoL demonstrates.

As the others have said, you don't necessarily need artistic skill to be a good level designer. Perhaps you should look into getting to know how to use unity and gain some technical artist skills. technical artists basically link the artists and programmers.

On 11/25/2017 at 8:12 AM, kane david said:

so who could do this part if it's hard i mean how they mange levels without level designer ?

From my experience, the team would get an idea of what should be implemented from the project lead, then the team gets together to draw up the levels together. The lead would then approve or not.