stupid question but..
is there a tut on how to make my maps look like something valve made? you know giving tips and how to design some of the things...
not just ONE tutorial. but if you follow alot of these tutorials and use prefabs. and just play the retail maps (ALOT) you will eventually start Thinking With Valve... also. since you haven't even played some of the advanced chambers i would say you need to play more portal 
Excellent point. I lost count of how many times I've played the retail maps, well not really played, more like wandered around looking at how Valve did it.
I'm not really sure what you mean by "look like something valve made" but a lot of what makes a map look "polished" is knowing how to use lighting and cube maps. I plan on reading up more on both of these, as I still have a lot to learn.
*COUGH*use vmex. Sorry my cold is quite bad.
The question was how to make something that looks like Valve's work, not how to steal Valve's work.
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It's not stealing if you don't actually copy and paste anything over. You can just use it as a reference. We're trying to make something look like Valve's work here so why can't we learn from the masters?
off of vmex. its been covered a million times. i personally prefer to do shit myself anyway. but yes even i break open the maps and look at them, just to get an idea for something that i wasn't sure of in the first place. but actually have only used the included "reference" map. (test chamber 5 i think it is) anyway. i've played portal ALOT. i mean alot. and its not just about doing things the way they did them. alot of the best maps probly only slightly resemble valve's (when compared in hammer) but they are just as good (if not better) its more about dedication to your product, to your baby, see Hurricaaane's Logic Portals those things only slightly resemble valve's maps. but they are great. there are many that i could think up but i'm to lazy right now to look up the url links.. so since i never half ass things you can check out Mapster's stuff, Duffedwaffe's stuff etc etc. these are both two more good examples of people who map in the valve style but obviously not following the valve [Religious Doctorine Literature Name Here]
anyway. the main thing is to get an idea of what you want your puzzles and map structure/pacing to follow. the look/design comes later. many people map in a different way, but i believe that those that make great maps (see the 3 listed above) are more concerned with the map being playable and THEN looking good; i'm to much of a perfectionist, which is why i haven't taken it upon myself to really start a mapping project yet, cuz i spend to much time making each room look perfect before i move on to the next, instead of making the game playble and THEN make it look good.
Just look at the portal maps a lot and try to figure out a set of design rules based on what you see.
Try to figure out the hard rules, the soft rules, and everything in between.
Examples:
- Floor textures are for the floors only.
- Wall textures are for the walls AND ceilings.
- Rooms that are mostly metallic tend to have white lighting.
- Rooms that are mostly portable tend to have blue lighting.
- There is an emancipation grid in front of end elevators, but not start elevators.
- Modular walls usually have smaller squares by the floor and larger squares by the ceiling.
- Etc.
If you want to make your map look like a valve map, it is important that you know what a valve map looks like.
I think I reached a comprimise with myself, instead of make each detailed room one at a time, or making the whole map and then adding detail. I make all the rooms and puzzles with a moderate amount of detail, some of the important lighting, key aspects of the detail but all the parts that are there just to make it look good (overlays, observation room props, glados nagging, etc) are left until the end.
Basicly I work on making all the unique parts first (the most important) including the puzzles (which should be unique) to make sure they play well then I pretty it up. We'll see how this works out soon with my current project 