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I saw this about seven hours after you posted it, and it took me some time to formulate a response, as battle maps just come naturally to me (most of the time; when I can't make my own, I tweak a published one). Here is my best attempt at advice, and perhaps others can elaborate further:

First, are you using 4 line per inch graph paper, or 1" grid paper? If you're using old school graph, that may be one problem. 1" grids help me a lot when I need to visualize my encounter. Just look up downloadable 1"grid PDFs, or use Incompetech's customizable system (all free). With the latter, change the size to 1 line per inch, and try the lowest possible line weight and a light color (to save ink), since you only need to barely see the grid. However, this may not be the problem.

Visualize the setting. A cave is a good place to start, since the shape is completely up to you. Rough-hewn walls for a naturally eroded cave, smoothe, symetrical walls for something carved by dwarves, or a combination of the two. Next, envision your environmental hazards and traps: lava pools, pits, falling rocks, cave-ins, etc., and place them strategically. Remember that you're not trying to kill the player characters, nor are you their babysitter. They need opportunities to use those well-earned skills to get out of trouble. If you're like me, you'll enjoy their successes as much as their failures.

Now, what inhabits your cave? Look into your Monster Manual, Monster Vault, whatever you use, and start off with level-specific creatures (using the "monsters by level" index), and find monsters that would inhabit your setting. If the characters are level 3, your creatures should span from level 1 to 5, level 6 for a boss. Be mindful of the ammount of monsters per level, for once again, you're not trying to kill them. Be sure and throw a mix of creatures at them to make it interesting. There are formulas in the books for building encounters, but it's all up to you in the end. 

Now, detailing your map. Let's say it's a cave that was once mined by dwarves, but hobgoblins killed off the miners and now use it as a base from which they orchestrate raids. Draw in the details of old mining carts and rails, a pickaxe and other tools lying about, maybe some tables with old parchment and flagons / plates, etc. Then draw in the crap that hobgoblins and their ilk may leave lying around: broken crates, refuse, dung-piles in the corner, bones, etc. Also, consider what each room was / is used for. There's an entrance, halls, mining areas, sleeping quarters, dining area, whatever. You may also have a final 'boss' area. You'll want to position it in way that the characters likely end up there last, but without railroading them. Maybe make an adjacent room equally accessible. In this 'boss' area, throw in some traps and hazards that make the final encounter a real challenge.

Finally, don't forget loot. Place it in hidden spots along the way, suitable to the PCs' levels, and don't forget to make it sweeter in the final encounter. You can also throw in an NPC somewhere along the way, or in the final encounter, to add story elements to the area, like a dwarf who needs to get out. This is all just an example, and if I've not made something clear enough, or completely missed the point, ask and I'll elaborate. I hope that others will offer their own advice as well. Good luck, friend.