Well everyone is different when it comes down to planning but planning - with everything - is one of the most important parts to get something going.
When you say you have been planning a game for awhile - have you actually put out a plan, so you know what its about, the atmosphere you want to bring and so forth? Or you are still in the "i want to make a game" phase and still thinking on the overall structure of it?
If not - write down some categories such as the genre (sci-fi, modern etc), Story/background (Where is it set, characters, setting, atmosphere), Purpose/Goals (What is the goal of the game), Style (FPS,RTS,etc) - and so on. Cut the planning categories into stages and when you have done that, you can go onto the next thing.
Then comes down to "how" you will make it - what experience do you currently have, are you are programmer, artist, musician - find out what you are good at and go then work around the flaws. What i mean by that will depend on how big your game will be - if its a small game like pong, tetris, small small, then being a programmer with bad graphic skills can still get away with a decent game depending on experience.
If you plan on a bigger more complex game, then maybe you have some friends who may be able to help out, spread the workload and skillsets to work together. Or find a team if the interest and commitment is there.
Overall there are many ways to go along with how to start - but you havnt really specifically said what you have at the moment so i gave a general idea for you to look at.
Well, the game i have planned is gonna be EXTREMELY open-ended.
I have been thinking about making randomly generated worlds, but problem is, i don't even know how i'm supposed to make the terrain into objects, and not just a plane with textures and models on it.
Kinda like Minecraft in that sense, where every type of block has it's own properties.
But like Minecraft, i'm satisfied with starting out with a pre-defined world, if i can just get a terrain that's comprised of several types of properties, then i'm sure i could go somewhere on my own.
So i guess what i really need is a way to change the terrain into a bunch of objects. (Dirt being an object, grass being an object, etc)
And now that i think about it, i have a feeling i just answered my own question, but please, do help me out in case i didn't.