Hello, and welcome to the modding community!
First off, if you haven't played New Vegas before, beat it at least once without any mods. This will give you an idea of what kinds of mods you want.
You can spawn all sorts of items ingame by using the console. The console is opening by hitting the (`) key next to the 1 key. Then you can type in a large range of commands that do various things. If you want something to appear ingame, you could try opening the console, clicking on a reference (like a table, or an NPC) then typing PlaceAtMe Blabla X. Where BlaBla is the FormID of the object, and X is the amount you want spawned. http://fallout.wikia.com/wiki/User:Deadlykris/FNV_Form_ID_List is a list of some of the FormIDs in the game.
If you are interested in creating mods with the GECK, head over to the wiki and do the Vault 74 tutorial. It's for Fallout 3, but about 90% of it's exactly the same. If you have any question, ask in the GECK section of the NV forum. http://geck.gamesas.com/index.php/gamesas_Tutorial_GECK_Setup
When you download a mod, generally you place the files in the Data folder (located in Steam/Steamapps/Common/FalloutNewVegas) then you have to activate them, either in the Fallout launcher, or by using the Fallout Mod Manager (FOMM). FOMM is a great tool, and I recommend getting it. It makes managing your mods and load order (The order in which the game loads all of your mods when you start it) much easier. Search for it at NewVegasNexus.com (Where you should get all of your mods).
Some mods can make the game slow. But it really depends on your computers specs and what kind of mod it is. Most mods that add new places can be laggy (Only in those new places!) if they are poorly optimized, but some Texture replacing mods can overall make the game laggy because the engine has to load higher quality textures. However, alot of people have computers that can handle these things so it doesnt make it laggy at all.
I hope some of this is helpful to you, and have fun with mods!
Alexander J. Velicky