First is learning how to use memory efficiently by instancing items from the same models. Most beginner designers just make one big model with everything. A developer might ask “What am I supposed to do with this?”. Game logic needs to move around all the items for the game to come alive. Save each model as one file, export and let the game load them in with information about how they are used.
Then you need to try level builders for actual games to understand how different representations work. Heavy indoor games may use portals or other occlusion techniques to master. Outdoor games often use height-maps or detail tesselation to compress the ground in memory.
"Far Cry 2" has a beginner friendly editor for creating height maps and a repeated vegetation pattern which is painted on a low-resolution map without storing individual tree locations.
"Unreal 2 The awakening" is probably the easiest game to make levels for with a simpler version of Unreal. This is both indoor and outdoor and can teach you how to make atmosphere with ambient sounds and light.
The first “Red Faction” game allow creating fully destructible levels, but you might need a mod to patch the level editor because of the bugs. In-house tools will likely have many bugs in early stages, so this is just a bit of realism.
You will have no chance to stand out from the thousands of job applicants unless you pay attention to both details and overall look and feel. Learn about what defines a style and avoid looking at cheap mobile games with random non-matching or generic store assets. It is likely that your resume will never even be looked at by the big studios and they will probably churn you if hired, so be prepared to work with friends for minimum salary or have it as a hobby that nobody can take away from you. Most people who get their dream job realize after a few years that it's just another job and the prestige means nothing. Even if you will succeed, another income will give you the safety needed to relax and stay creative. Nothing ruins artistic ability like knowing that your basic needs depend on it. If it makes you happy either way, then go for it.