I find quark simply confusing and it has(which admittedly could be very usefull) a huge amount of options that make it a whole lot more newbie unfriendly.
Doesn't gfscape need .net framework though? You'll get this weird and non-explanatory crash message with some memory adress trying to run a program that wants the .net framework but it isn't installed.
A few things:
Select view-> 3d textured polygons or you won't see how the texture is applied on your brush in the 3d view and if you can't see that you can't figure out how the texture application tool works by playing around with it.
Rooms in half-life are built by puting a bunch of brushes togheter so that no leaks to the outside 'void' occur. To make a simple box room without using any additional tools, make a hollow box out of 6 block brushes and texture the walls to your liking. The walls to the outside void do not need to be thick, along the lines where brushes meet it needs no thickness at all as long as the faces on the inside leave no gaps. Brushes need a thickness, it doesn't matter if it's 1 or 233 but it is easiest if it is a nice big round number such as 16 or 32.
Select a bunch of brushes, hold down shift and drag and they will be copied to where you drag them(This is hardly usefull unless you have texture locking on(button with "tl" on so that you won't have to keep reapplying the textures)).
You can group a bunch of brushes so that you select them all at once instead of individually. Do this by selecting them and pressing the green dotted square icon with the 2 brushes in it. To ungroup press the 2 brushes being snapped apart looking icon.
"ig" means ignore group so that you can pick out and edit a single brush in a group of brushes without ungrouping them.
You can split and chop of pieces of brushes as you wish with the clipping tool, select the brush/es and then select the clipping tool and click and drag where you want to cut. Press the clipping tool a few times more until you get the mode you want(split, chop of one of the sides of the line, chop of the other side). Be carefull to avoid having vertices that do not lie on a grid coordinate(the compile tools will force smaller detail onto the grid, often with bugs as a result). To check if a vertex is not on the grid use the smallest grid size and zoom in on it.
ctrl+m gives you the transformation tool, this allows you to rotate, scale and translate objects(use texture locking if you care about retaining the textures the way they where). Rotation and scaling is likely to cause vertices not on the grid. Rotating 90 degrees or 180 degrees in any direction should allways work without giving vertices on the grid. Translating with whole units should allways work. Scaling up with an integer should allways work. Scaling down with 2 should allways work if the vertices lie on the grid when you are using grid size 2, scaling down with 4 should allways work if the vertices lie on the grid when grid size is 4 etc. Smallest grid size is 1 unit.
Textures are allways of size 16*integer in each direction, keeping geometry simple and evenly divisible with 16 is usually a good idea. Textures are split to w_polys every 224 units if tiling and 240 units if not tiling. If you scale your texture by a factor of x(non-integer allowed) splits will occur at x*224(rounded to integer) for tiling and x*240 for non-tiling. Scaling up a texture on a face could therefor be very beneficial for your r_speeds, typically in large open areas. Scaling down a texture can be of no consequence if the face is small.
Polygons must be flat, if you use the vertex editing tool(allows moving "corners" of brushes) you will very likely end up with a faulty brush unless your carefull. A good thing to remember is that a polygon with 3 sides can be manipulated in anyway at all without being invalid, this is because you can allways find a plane such that the 3 vertices lie in it, this isn't true when you have more vertices. Since you won't be dealing directly with polygons but with brushes there are no triangles but only 3d primitives, however you can make a brush with only triangles as sides and the only thing you could possibly do to mess that brush up with vertex editing is turn it inside out. The most usefull application of this is taking a block and spliting it from the top along the diagonal with the clipping tool(press the clipping tool several times to get to the split function, the brush need to be selected to be clipped or it won't know what it is supposed to apply it to. You can clip many brushes at once) and then you can move the vertices up and down however you want without the brush being invalid, this is usefull for terrain and outside areas; there are problems, the clip hull(collision for players) gets somewhat buggy in many places on such terrain(HL or the compile tools doesn't like it when the normal of a surface doesn't lie in the plane defined by any pair of the coordinate axes in the editor and the collision with such a surface is often slightly bugged), and HL isn't good with handling many polygons so don't go overboard.
You need to place entities in your map, such as player spawns, lights, hives and a comm chair, there are excellent guides for the entities NS needs to have and how it all works.
Carve is evil, carve is evil, carve is evil, carve is evil, carve is evil, carve is evil....
ctrl+i and ctrl+l are usefull for mirroring a bunch of brushes(mirroring is not the same as rotating 180 degrees as anyone who has played tetris will allready be familiar with

).
All faces need to be convex, it's not nessecary to undertand what this means but as an effect where a complex object meets a flat surface is going to look like a mess of odd angled w_polys if you don't make the object(typically a pipe) a func_wall(entities don't "split walls") or if you don't leave a small(1 unit gap). To see what your w_polys look like in the game type "sv_cheats 1" and "developer 1" in the console then type "map <mapname>" and finally gl_wireframe 2 (you see a wireframe of polygons even through walls, this is what the engine see's and renders which is usefull to know so that you can place VIS blockers and hint brushes(a more advanced topic, don't worry too much about this until you have gotten a hang of the basic tools) )
Alot of newbie mappers make huge untextured rooms and don't use texture lights. Learn how to use texture lights(google up a tutorial, shouldn't be hard), try not to make huge empty spaces they are boring, no official map has anything but a few open spaces, corridors of sizes 128-256 units in width are pretty descent sized and easily traversed by everyone including onos if they are taller than 72 units(128-256 is to be recommended there as well).
If you don't use the nshulls.txt files in your nsp directory as a custom hullfile you will get weird stuck issues with the onos, it is more important that you get a hang of making simple maps first before taking on all the NS specific issues.