Custom Classes Overview
Custom classes are where the Elder Scrolls games really shine. They allow you to create the character that YOU want, not the character that the developers think you ought to have. Here are some random thoughts on developing your custom character.
1. For a quicker-leveling character, play to your strengths. Choose your most frequently used skills as your Major skills, choose a specialization that matches those skills as closely as possible, and then play to those strengths. For example, warrior-types won't sneak around or do much spell-casting, aside from the inevitable Restoration spells, so they can leave those skills in Minor and focus on other things. Since you want to gain levels quickly, put easy-to-develop skills into your Majors. So, for a quick leveling Warrior-type, take a Nord with a Combat specialization and a Warrior or Lady birthsign. Choose Blade, Heavy Armor, Block, Acrobatics, Athletics, Armorer and Restoration (cuz you'll need it) in your Majors. Pick up a one-handed longbladed weapon with a shield, get some heavy armor and buy a boatload of armorer's hammers. Now run/jump everywhere you go, fight everything that crosses your path (use your Restoration spells to heal up afterwards) and be sure to repair everything after every fight. The higher Strength and Endurance should allow you to carry quite a bit of loot, but don't expect to get the best prices when you unload it cuz you've got a personality that only a Goblin could love (and they're not too fond of you, either).
2. For a slower-leveling, but ultimately more powerful and versatile character, play to your weaknesses. Leave most of the skills that you're going to use frequently in your Minors and put useful, but not frequently-used skills into your Majors. Because you're not using these skills very much, you'll most likely have to train a lot in order to gain levels. But on the other hand, you're not going to be facing the higher-level opponents as quickly and when you do run into leveled opponents, your strong skills should be much higher than theirs. For example, I like stealthy spellcasters, but I rely much more heavily on stealth than on magic. So, I leave my Stealth skills in Minors (Sneak, Security, Light Armor, Blade, Mercantile, Speechcraft, etc.). I choose something like a Breton or a High Elf (for the extra Magicka) with a Thief or Warrior birthsign (cuz I'll take all the help I can get on those low starting stats) with a Stealth specialization, and put Marksman, Illusion, Mysticism, Armorer, Blunt, Restoration, and Acrobatics into my Majors. This combination gives me low, but workable starting skills on the skills that I'm going to use most often and higher starting skills on the useful skills I'm going to use less often. The nice part about those particular Majors is that they are relatively easy to practice when I decide it's time to level up (Marksman and Blunt being the two notable exceptions). Oblivion, like Morrowind, gives attribute-point multipliers regardless of whether the skill is in your Majors or not, so I should be in good shape when it's time to level up because I've still covered all seven attributes in my Majors.