"Can I fight battles with my own created chapter? or do I have to follow a specific chapter, even though I can paint them how ever I want. I have the New Codex for Space Marines and understand the points, swapping wargear, can only have so many heavy arty, assault troops, Tact. squads, HQ's, elites, stuff like that but when you say one is better at something then the other I'm lost. Wouldn't the make up of your army/points make that distinction, not the chapter? I have made a Chapter called the Blood Marines, and written a brief bio and history of how they formed, but like the ranks, and insignias stuff like that, do they all follow the same format? I have a Terminator Librarian I will use for now as my HQ, but can I give him a name, for instance one that would suit my story line and his role as so and so, Commander of the Blood Marines? I just don't want to do all this and find it's not viable in table top games."
I get questions like this a lot. And this is pretty confusing to new players just coming into 40k. Here is the answer. If you want to play Space Marines, you have a few options to what rules you want to use to create your army. Codex: Space Marines, Space Wolves, Blood Angels, Black Templars, Dark Angels and Grey Knights are all "Space Marine" chapters. It is sort of like the US military. We have the Army and we have the Marines. They are all military, but don't you dare tell a marine he is in the army! Don't call a Navy Seal a Green Beret.
The generic book is Codex: Space Marines. The BA, BT, DA and GKs are all specialized chapters. The difference is that while armies created using rules from Codex: Space Marines are more versatile and have the most amount of options, the other codexs specialize and have restrictions on what they can take in exchange for powerful abilities not available to Codex: Space Marines. The Blood Angels for example can take Jump Pack Assault Marines as troops. Dark Angels can take Terminators as troops. Black Templars have special rules that enhance their troops called Vows. Grey Knights specialize against Daemons and have lots of psychic powers. But all of these chapters give up things that the Codex: Space Marine armies have. C:SM armies have Combat Tactics and other army wide unique special rules not available to the rest of the Marine chapters. C:SM have special gear like Thunderfire Cannons which are very powerful and cool special characters like Vulkan.
So, if you want to play your own army you need to do two things. First, decide on a theme and color scheme for your army. You have done that. Second, decide how you want them to play. Then you pick the codex that best suits your purpose. If you want to play all of the great Space Marine toys, choose Codex: Space Marine. If you want to play lots and lots of fast moving Assault Marines, play Codex: Blood Angels. The only rule is that you must pick a codex and follow the rules from that book to create your army. Color, names, background are all up to you. But you have to follow Force Org Chart rules from ONE book only.
Blood Angels or Black Templars?
BA or BT? Well, first of all, you need to decide what colors you like. Black or Red. But there is more to it than that. I won't get into the competitive nature of the codex books. BA have a new codex written for 5th edition. BTs have a 10 year old book designed for what was really a different game. BT cannot field a tournament competitive army as good as BA. I'm sure that will be debated, but I need only point to the top ten finishers at all GTs in the US this year and you will see many more BAs than BTs.
Having said that, both armies focus on assault. They want to get down and dirty. Both have unique options for troops. BA can take assault marines with jump packs as troop choices. BT can take 10 man squads of marines with assault weapons, and compliment them with 10 more scouts, or Neophytes, for huge 20 man squads. Both armies have powerful characters, though I have to give the edge to the BAs for sheer ferocity with characters like Mephiston. Both have army wide special rules that make them more powerful. BA can get furious charge, fearless and feel no pain for every marine in the army. BT can get Preferred Enemy and fearless in close combat and leadership 10 for every marine cheaply. The two armies are very similar, but very different.
Part of your decision should be fluff. What do you like more? Space Vampires or Crusading "Dark Ages" knights. They have unique backgrounds and motivations. The chief difference is that the BA were a founding chapter, one of the first 20 or so Space Marine chapters that fought in the Horus Heresy. The BTs are a Second Founding chapter, actually an off shoot of the Imperial Fists, which was created after the Horus Heresy because the Space Marine Legions were just too large and needed to be broken up. My advice? Get the books and read them. Pick the one that most appeals to you, and use it. :)