You get the HD/HP of a Fighter 11, and the skill points from that Fighter level, and you get any class features of a Fighter 11. Just not the BAB and base save increases. Of course, 11th-level Fighters don't normally have any class features, so you'd just get the HP and skill points.
Now if, at your 22nd character level, you went up to Fighter 12/Wizard 10, you'd get the HP and skill points again as a 12th-level Fighter (1d10 + Con mod, and 2 + Int mod, respectively), and you'd get the bonus feat a 12th-level Fighter receives.
Similarly, if at 21st-level you instead went up to Fighter 10/Wizard 11, you'd get the HP and skill points of a Wizard 11, and the new spell slots, spells known, and any increase in familiar benefits normally granted at Wiz11. But not the BAB and save increases, of course.
Then at CL 25 for instance, if they went up to Fighter 10/Wizard 15, they would get the bonus feat acquired by Wizards at 15th-level, as normal. Plus the HP, skill points, spell slots, spells known, and familiar increases for a 15th-level Wizard.
Once your Fighter level reaches 21, then you'd start to accumulate bonus feats from the Epic Fighter list, at the rate noted for Epic Fighters, and of course you get the HP and skill points from that level as normal.
Similarly, once your Wizard level reaches 21, then you'd start to accumulate bonus feats from the Epic Wizard list, at the rate noted for them in the Epic Level Handbook (or SRD, or 3.5 DMG?), as well as the HP and skill points. But as noted in the ELH, spellcasters don't accumulate any more normal spell slots at epic levels (when their spellcaster class level is 21 or higher). They have to take Epic feats to do so, and to acquire Epic spells if they so choose.
NOW, separately...... Once you reach 21st character level, you can start gaining Epic feats, if you meet the prerequisites for them. You also accumulate an Epic Attack Bonus, and an Epic Save Bonus. Refer to pages 5 through 8 in the Epic Level Handbook for the information you need. Basically, you get a +1 Epic Attack Bonus at each odd-numbered Epic character level, so 21st, 23rd, 25th, etc. You gain a +1 Epic Save Bonus at each even-numbered Epic character level, so 22nd, 24th, 26th, etc. Note that it's your total character level which determines this stuff, rather than your individual class levels.
Your BAB and base saves do not increase at Epic character levels. Ever.
Instead, your Epic Attack Bonus is applied to your BAB for two purposes, ONLY:
1) It applies on your attack rolls of course, and
2) It counts towards your effective total Base Attack Bonus for the sole purpose of meeting prerequisites and such. So, a Fighter 25 could use Power Attack for 25 points, and could gain feats that had a prerequisite of "BAB +25".
Your Epic Attack Bonus does not apply for purposes of gaining additional attacks. Thus, if you have normal BAB of +15/+10/+5 before you reach Epic levels, you still only get 3 attacks with a full attack. It doesn't matter if your character has reached the lofty heights of level 100 in the Rogue class, you still have only 3 attacks per round. If your character is a straight Fighter 100, he still has only 4 attacks per round, same as he did when he was a 20th-level Fighter.
Similarly, if your character is a Fighter 10/Wizard 10, then reaches Epic character levels, he will still only have a BAB of +15/+10/+5 for purposes of determining his number of attacks. He may become a Fighter 40/Wizard 40 eventually, but that just means his attack bonuses are +45/+40/+35, and he still has only 3 attacks per round, same as he did before Epic levels.
YES, this screws anyone less than a pure warrior-type, nor nearly so, when it comes to number of attacks per round. A Fighter 12/Wizard 8 (total BAB of +16/+11/+6/+1) would be the most you could multiclass, for instance, pre-Epic, if you wanted to possess 4 attacks per round at Epic levels.
Thus, for such a character, you'd be better off going Fighter 4/Wizard 6/Eldritch Knight 10 (since this build would give you 20th-level BAB of +17/+12/+7/+2, and base saves of +13/+6/+9, which of course would never increase any further once you go Epic-level). This would give you 15th-level wizard spellcasting by 20th. Then at Epic levels you could further increase your Wizard levels for more spellcasting.
Or your EK levels, but that won't advance your familiar, and it gives bonus feats at a slower rate once you get to Epic Wizard levels (Wiz level 21+), just for the paltry difference of d6 hit dice to the Wizard's d4s. At Epic levels that's not likely to matter. I think the 3.5 DMG or something else has the Epic progression for Eldritch Knights; I just know that it's at least in the Hypertext SRD on the internet.
Now, once you improve your level to Fighter 4/Wizard 11/Eldritch Knight 10, you'll have the spellcasting of a Wizard 20. Further levels of Wizard after that point won't give you any more spell slots, but you will continue to accumulate more spells known of 9th-level and lower. You'll have to take Epic feats to gain further spell slots, or Epic spells.
Of course, your familiar will be equivalent to that of a 6th-level Wizard at your 20th character level (since Eldritch Knight doesn't advance familiars), and will improve to equal an 11th-level Wizard's familiar once you max out your normal Wizard spellcasting in this way. However, additional Wizard levels will continue improving your familiar's benefits.
However, note that your familiar will still always have half your maximum hit points as its own maximum; a high-level multiclass Wizard will have a really tough familiar by comparison to a single-class Wizard. And it will still be treated as having the same number of Hit Dice as you have. So once you are a Fighter 4/Wizard 21/Eldritch Knight 10, your familiar is treated as having 35 hit dice, though it's unlikely to matter for most purposes (still, some offensive spell effects are only able to affect a certain number of hit dice, at least).
Once you are a Fighter 4/Wizard 20/Eldritch Knight 10, you'll have the spellcasting of a 20th-level Wizard, but an effective caster level of 29 for purposes of spell effects and such (just not for spells per day, since normal spell slots don't accumulate automatically past caster level 20). However, at that point your familiar will have the benefits of a 20th-level Wizard's familiar. Once you advance to Ftr 4/Wiz 21/EK 10, your familiar will gain the benefits of a 21st-level Wizard's familiar. Familiars do continue to advance in benefits beyond Wizard 20.
Hopefully not too confusing...
