you will need the broken steel DLC to reach level 30.
first thing's first.
start a new character, and adjust your SPECIAL how you see fit, but making sure that your INT is at 10. for your 3 tag skills, i would suggest small guns, medicine and repair. just to make your life a little easier until you're beastly. remember to snatch the medicine bobblehead from your father's desk, and then make it through the rest of vault 101 until you get outside. (sorry livewire)
once outside, you will level up. you will have 20 points to distribute amongst your skills, and i suggest you spread them out, keeping your skills as even as possible. next stop is megaton, which is SE of you. once there, you'll want to find walter, who spends his time at the water processing plant. not too hard to find, but here's a map.
http://fallout.wikia.com/wiki/File:Map_over_Megaton.jpg
speak with walter, and tell him that you'll patch up the 3 leaks around town. 2 of the leaks are pretty easy to spot. the 3rd can be a little harder if you've never done this before. here's a 2 minute video with the locations.
http://www.youtube.com/watch?v=9tbsIlOO_cI
from this point, you're going to want some scrap metal and a stealth boy. 16 pieces of scrap metal seems to be the magic number, but apparently any even number will work. the more the better though. i headed directly south to the nuka cola plant and found 6 pieces of scrap metal and a stealth boy, but such a low number proved to make the leveling process MUCH slower.
either way, obtaining 16 or so pieces of scrap metal shouldn't be too hard. another place to consider for metal and a stealth boy is the robco facility, located to the SW of fort independence. thanks to NEMESIS MASK for that tip.
here's a map of the wasteland.
http://fallout.wikia.com/wiki/Fallout_3_map
12 is vault 101 (where you start)
K2 is megaton
S7 is the nuka cola plant
09 is fort independence
R5 is the robco facility
once you get enough scrap metal head to fort independence and make friendly with the outcasts. save your game and prepare to speak with protector casdin. this is where the scrap metal glitch comes in.
http://www.youtube.com/watch?v=MfzJNsJn9sw
if you followed the 4 minute youtube guide, you will now have unlimited scrap metal in your inventory, but you won't be able to see it. and thankfully, it won't weigh anything either. you can test it by speaking with casdin again, collecting the reward of your choice over and over again. stimpaks, 5.56 rounds (nice!) or whatever else.
DO NOT pick up any more scrap metal. ever again!
doing so will apparently void the glitch.
if you did it correctly, it's time to fast travel back to megaton. speak with walter again and tell him to take all the scrap metal you have. you will get a large chunk of EXP and bottle caps, and you can do this over and over until you hit level 30. starting the glitch with 6 pieces of scrap metal, it took me 90 minutes of pure button mashing to reach level 30. with 16 or so this shouldn't take you any more than 30.
you will be leveling very quickly, especially early on. here are a few tips:
at level 4, take the educated perk
at level 5, take the comprehension perk
don't waste perks. don't bother with the daddy's boy/girl, gun nut, here and now, intense training, iron fist, little leaguer, scoundrel, size matters, swift learner, or thief perks. these are all completely useless. during the first couple levels you might have to pick one or two of these until other ones start to unlock. no real worries though.
when leveling, try and keep all of your skills fairly balanced.
remember that there are karma based achievements at level 8, 14, 20 and 30. if you don't have these achievements, you might want to head outside the plant and adjust your karma. if you've followed this guide, you should have neutral karma... unless you had a little fun on the side. either way, the temple of the atom is very close to you, and you should have more than enough caps by now to blast your karma through the roof via donations. if you want to lower it, go on a stealing spree. or kill some people... though you might want to research who you off.
anyway... bam. you're at level 30. and you have one more perk to pick.
you're going to choose the "almost perfect" perk, which bumps all of your SPECIALs to 9, with the exception of INT, which you already have at 10. so now it's time to do a little exploring. you have 9 bobbleheads to find to max your SPECIAL.
http://fallout.wikia.com/wiki/Vault_Boy_Bobblehead
the top list is what you want. the one in rivet city (INT) isn't necessary.
after you've collected these, you're done with your SPECIAL, and most of your skills should be pretty close to 100 already. all you have left to do is hunt down some skill books to top em off. and because you chose the comprehension perk, the books you read will increase your skills by 2 points instead of 1. here are the locations of the books.
http://fallout.wikia.com/wiki/Fallout_3_books
and that's it. the perfect character.
NOTES:
july 28th 2009 - i've gotten a character to level 30 in 2 1/2 hours game time.
1 1/2 hours of the time was nothing but spamming the A button. if i had used the recommended 16 pieces of scrap metal, i could probably knock an hour off of this time. at level 30, with all SPECIALs at 9 (INT at 10) all of my skills are at 80 or higher. once i've gathered the SPECIAL bobbleheads and the required skill books i will update the guide with my final time count. i imagine it will be around 5 hours total for the perfect character.
the scrap metal glitch has not been patched.
august 16th 2009 - i've never encountered this, but you may need to get on casdin's "good side" before you can trade your metal to him. if this is the case you'll need to find an energy weapon or some energy ammo i believe. or you can run to the alien crash site and snatch the alien pistol and power cells. the crash site is U1 on the map, which is far NE. thanks to Drunk3nDuffman for the info.