The first feedback I got on this web site was "If someone was contemplating starting a garden railroad, I don't know if this would be encouraging or discouraging..." Before you decide, first let me assure you that I blatantly disregarded all advice to start small. And here are a few other tips you may find useful.
Tip #1: You do not have to start outrageously huge. Most people start small and grow. You do not need to put your railroad on a raised bed, and you do not need to start with mountains in your layout. And a lot of people start with absolutely no plan at all.
Tip #2: Track power works just fine. You do not need to convert your locomotives to battery power like I did. If you do want to do any of the conversion stuff, be sure you have at least a little electronic technician in your blood, or you are good friends with somebody who does.
Tip #3: You can still do wireless control with track power. If you like the idea of a handheld remote to run your layout, but don't want to go to the trouble of converting the locomotives to battery power, you can get track power base stations that receive the wireless commands, but run track power.
Tip #4: You do not need to retrofit the controls at all. You can buy non-DCC locomotives that will simply move faster when you apply more voltage to the track, which is something the most basic track power control will do (and was the only way it used to work). You can also buy locomotives with DCC already installed. These will run on track power, but take their commands from the track. Traditional DCC (as opposed to wireless) transmits the control commands electrically via the track right along with track power. You may want to look for a controller that matches the brand of DCC already installed in your locomotive, but in theory, you don't have to because DCC commands are standardized. If you are contemplating a mix of manufacturers, try to find somebody who has already tried that combination just to be sure they are really compatible. The commands are supposed to be standardized, but I have not personally tested various brands for compatibility so please don't take my word for it.
Tip #5: Learn about track. I am using Code 250 flexible aluminum track with plastic ties (from Llagas Creek). The "code 250" means 1/4 inch track height, and code 250 is not the only commonly used size. While the spacing between track is standardized at 45mm for G scale, the height of the track is not standardized. Once you settle on a "code", you will want to stick with it. Also, the construction of the track, even when "code 250", is not the same from one manufacturer to the next (unfortunately). So again, pick a manufacturer and try to stick with it. I prefer plastic ties over wood simply because they will stand up better outdoors. I also mentioned I am using flexible track. That means all the track is initially straight sections, and I have to bend it to match the curves in my layout. It is really more common to go the other direction - buy preformed curved pieces, and build the layout more according to how the track fits. If you're not sure about brand and want to start somewhere safe, you can't go very wrong with LGB.
In addition to code, the other thing to pay attention to is "standard" versus "narrow gauge" track. The same code 250 track will look drastically different between standard and narrow gauge. The ties on narrow gauge track will be bigger and spaced further apart while ties on standard gauge will be smaller and closer together. The rails are 45mm apart either way, but they are intended to simulate standard or narrow gauge railroads. On a real life track, narrow gauge rails are spaced 3' apart, while spaced at 4'7" for standard gauge. Therefore, you have to play with the tie size and spacing on the model track for visual effect or proper perspective. Everything in my layout is narrow gauge, and all the locomotives are steam engines from the early 1900's.
Tip #6: Learn about couplers - the things that connect the cars to each other. The most common and most interchangeable is the "hook and loop" coupler. As far as I know, the only thing anybody has against hook and loop is that they do not look as realistic. I don't believe they are less reliable than anything else. I personally have "knuckle" couplers on all of my locomotives and rolling stock. These are the more realistic looking ones. But what I learned the hard way is that there are at least 4 different manufacturers of knuckle couplers, and none of them connect to each other reliably (or at all in some cases). My collection started out with a number of cars from Bachmann, which came with Bachmann knuckle couplers. I ultimately ended up retrofitting everything else with Bachmann knuckle couplers. You can buy just the couplers, and I bought a lot of them. If you want to start easy, stick with hook and loop, which is standard on most LGB equipment. Bachmann cars come with knuckle couplers pre-installed, but they also come with a spare set of hook and loop couplers for you to swap in if you prefer.
Tip #7: Learn about scale. I originally thought "G scale" meant everything was built to the same scale and would be interchangeable. Not so. On the "big" end of the range, you have Bachmann's Spectrum series which is 1:20.3 (along with some Accucraft). The most common is 1:22 and this is the scale LGB and several other manufacturers use. Then there is 1:29, which USA Trains and a handful of others are. If you look hard, you can even find 1:32 scale, still called "G" scale. I have both 1:20.3 and 1:22 on my layout, but avoid 1:29 just because all mixed together, they just don't look right. The other thing I found out is that the coupler height on 1:20.3 is higher (and not compatible with) 1:22 scale. But they thought about that ahead of time - at least some 1:20.3 cars come with a second set of couplers you can swap in to make them compatible with 1:22, which is what I've done with all of my 1:20.3 cars. The other thing you'll eventually want is buildings, automobiles, and trucks. These are commonly available in 1:24 scale, which will be close enough to your train scale to look ok regardless of which brand railroad equipment you've got.
Just for reference: G scale is 1:20.3 through 1:32 (most common is 1:22), O scale is 1:48, HO scale is 1:87, N scale is 1:160, and Z scale is 1:220. You might also see reference to Fn3, another (maybe more proper) name for 1:20.3 scale. You may also see On30 which is O scale cars and locomotives running on HO track.