Sliders? No. Never use them. Thrusters? Not very controlled. See, to make a door, you need ballsockets and a Hydraulic.
Anyway, you need:
-PHX (SVN) A model pack that is very well known for its plates of varying sizes. Allows very customizable building, and the SVN link also gets smartsnap and easy precision, I think. Easy precision is an alternative to ball socket, easy weld, and axis. Smartsnap draws grids on things when you look at them with a toolgun that you can snap to points using the E button.
-Wire (SVN) Makes the game have more stuff that you can make with basic programming syntax, as stated above. The syntax takes place in forms of function chips, laser pointers, pod controllers, and other things you just stick together with the Advanced Wire tool. Especially Expression 2. That's pretty much just programming, though.
-Stacker. Makes a copy of the prop you click in with the settings you choose, so it can be frozen, welded, nocollided, or any combination of the two. Oh right, and the copy is moved of to the direction you specify.
That's pretty much all you absolutely need. SVN links are here:
http://wiki.garrysmod.com/?title=SVN_Links
If you don't know SVN, there's a pretty easy way to figure it out.
http://wiki.garrysmod.com/?title=Svn
I prefer just using Tortoise SVN to manually download addons, but before I did that I liked using EasySVN.
http://easysvn.zetorium.com/
Unfortunately, at this exact moment in time, it looks like EasySVN is down for now.
Anyways, the stuff I mentioned can all be used to make a working door, but wire only gives the door more options. But, if you would like to know how to make a door, just ask.
If you want addons just for fun, I suggest:
--Stargate addon. I knew about it before I watched the show, I still liked it. It gives you several devices from the show (but the asgard beam entity can only be used with wire) including the gates themselves. Some people have started making their own kinds of addons for the addon itself(Stargate Weapons in the Facepunch forum's Lua Scripting/RELEASES/Scripted Weapons section, Stargate Addons in the Facepunch forum's Lua Scripting/RELEASES/Addons section. Stargate itself is in the Lua Scripting/RELEASES section.)
tl;dr: It adds a bunch of portals, but the asgard teleporting thingy only works with wire.
--Spacebuild. A gamemode where the goal is to colonize other planets in space. This is one of my favorites, as it keeps the creativity of Garrysmod while adding a challenge for the player. It goes really well with the SBEP, and it requires LS and RD to work. If you're getting Spacebuild 3, you need LS3, RD3, and CAF(Life Support 3, Resource Distribution 3, Custom Addon Framework) If you just want Spacebuild 2(simpler) get LS2 and RD2. CAF is not needed for SB2.
tl;dr: Colonize planets with SB(2/3) LS(Same #) RD(Same #) and CAF(If you have SB3). Below is almost necessary for this.
--SBEP. The SpaceBuild Enhancement Pack. This adds a TON of models that can be Easy Precision Welded to stick on to each other to make a ship. Or you could use regular easy weld. It also adds a bunch of entities, like pre made ships and guns. The guns require GCombat to work. I should warn you, the link on the SVN Links page is not the newest version, nor the best. The newest one includes a prop spawner tool and a ship assembler tool, which spawns SBEP props with a hardpoint that you click on and then another of a different prop to move it over there and either weld it or just freeze it, depending on the checkboxes.
tl;dr: Model and Entity pack that makes Spacebuild easier by having things you fit together with welding. But the version on the SVN Links page on the GMod Wiki does not have the newest, simplest version.
--GCombat. A system that makes props destroyable, and adds the GCombat tool, which makes guns. Needs wire, though.
That's about all I have to say right now. If you want more addons, just search for terms that might be interesting on garrysmod.org or wait for a featured addon to pop up on garrysmod.com By the way, I haven't mentioned any maps, but custom maps do exist, like the freespace series (freespace_revolution all the way from freespace_1) Also, there are many more gamemodes that people have made, like DarkRP and various Fretta gamemodes(Fretta is an official gamemode base included with Gmod, don't worry about it.)
tl;dr overall:
Click on the links, get PHX 3 and Wire, go to garrysmod.org and search stacker, get that, done.
[Edit]
I am
the master at overcomplicating things. I am
a semi-master at Gmod. I guess that's why I love E2 so much...