A question of Hit Dice

[Smoof]

Alright, so I'm making a new character for some 3.5 D&D and I've decided to go with a Druid. I was looking at the stuff for animal companions and it tells me they increase in hit die as I gain levels.
3-5 +2
5-7 +4
etc
It says I not only have the animal gain in hit points, but also base attack and saves. How does this work? First, does the animal just gain +1 at level 3 and then +2 at level 4? Or upon hitting 3, does it just gain a straight +2 to hit dice at 3 and then no more until level 5? Then, how does it work to apply that to base attack and saves? Does it just have a +2 to base attack and saves now? Do I add it every level? As in, +2 at 3, +2 at 4, +2 at 5?

[Nameless]

Short answer because I have to leave in 5 minutes: The bonuses happen all at once as soon as you qualify for them, so at 3rd level, your companion gets +2 hit dice.

These hit dice are full HD for the creature that is gaining them. To find out how to advance your companion, you can check out the back of the Monster Manual, which has rules on how to advance creature's HD. If you have any more questions, post them and I'll try to answer them tonight!

[J Arcane]

HD are essentially treated like levels for the purposes of monster BAB and saves.

There's a section in the MM that gives you what those bonuses are for each type of monster. BAB is a function of HD, and will be either Full HD, 3/4 HD, or 1/2 HD, while saves will be described as Good or Poor, and there's a table telling you what the actual numbers will be.

[Karak]

Alright, so I'm making a new character for some 3.5 D&D and I've decided to go with a Druid. I was looking at the stuff for animal companions and it tells me they increase in hit die as I gain levels.
3-5 +2
5-7 +4
etc
It says I not only have the animal gain in hit points, but also base attack and saves. How does this work? First, does the animal just gain +1 at level 3 and then +2 at level 4? Or upon hitting 3, does it just gain a straight +2 to hit dice at 3 and then no more until level 5? Then, how does it work to apply that to base attack and saves? Does it just have a +2 to base attack and saves now? Do I add it every level? As in, +2 at 3, +2 at 4, +2 at 5?

They should happen all at once if you are doing standard rules. If you are doing extended rules(less like a video game and more like true roleplaying) they happen lowest gain first and so on at a DM determined and game determined level.

[Smoof]

Alright, so I couldn't find shit about improving base attack and everything in the Monster Manual. It didn't explain advancement beyond gaining more HP.

[nnanji]

If you look in the glossary under the creature's type (eg: animal type, abberant) it will define that type's characteristics. This will tell you how a creature advances. How to calculate BAB, which save types are good saves, etc.