OK, firstly.. this isn't a simple or pleasant game.. (its also well old being near enough a launch title!)

The game works using a very tight Counter attacking mechanism, sadly though the timing window for these counters are so small its near impossible.. which makes the game ruddy impossible..
The game also 'stores' your previous fighting moves and so will tend to block/counter them more as you use them.

The worst thing, it will give you an achievement fairly quickly in the game and so you'll be left with an awful score from it.

This is just for the Single Player achievements.. The Multiplayer require different methods.

Firstly, keep a record on Paper/Excel/Note of your progress.. it makes it much easier.

Play the Story Mode through to completion as each character.. Yes, its shit, and the computer is cheap. but it can just be persevered through..
There are 16 selectable characters to play through..
You will fight 8 different enemies but you only have to win 1 bout with each one but can restart when you lose..

Complete the Story once and you will unlock 'Completed Story Mode' 10GS

As you progress with each character you will also unlock the following :

Complete the Story mode with Hitomi and you will unlock Ein 'Unlocked "Ein"' 10GS
Complete the Story mode with Elliot and you will unlock Gen Fu 'Unlocked "Gen Fu"' 10GS
Complete the Story mode with Zack you will unlock Leon 'Unlocked "Leon"' 10GS

Unlocked characters do not have a Storymode (except Helena)

When you have completed the Story mode with all characters you will unlock Helena and get these achievements:

Complete the Story Mode with all characters : 'Unlocked "Helena"' 20GS

Now, Complete the Story mode with Helena and you will get these two achievements:

Complete Story as Helena : 'Unlocked "SPARTAN-458"' 20GS
Completed All StoryModes : 'Completed All Story Modes' 30GS

Now, go into Options and change the Time Trial match Point to 1 (on both Single & Tag)
This makes the Time trial like the story mode - you only need to win a single match.

Complete Time trial for all available 21 characters and you will unlock 'Unlocked "Tengu"' 20GS

During all this fighting for Time Attack, You may have unlocked the following (its pure luck anyway!)
Bronze in Time Attack (Single) 10GS
Silver in Time Attack (Single) 20GS
Gold in Time Attack (Single) 40GS

Pick a couple of characters you wheren't too bad at and try the same with Tag Time Attack
Bronze in Time Attack (Tag) 10GS
Silver in Time Attack (Tag) 20GS
Gold in Time Attack (Tag) 40GS

There are Time locked achievements too.. This count count spent in matches, not menus..
'Total Play Time 5 Hrs.' 10GS
'Total Play Time 10 Hrs.' 20GS
'Total Play Time 100 Hrs.' 40GS

You can unlock these also by leaving your 360 on whilst in 'Sparring mode' - The achievement unlocks when you exit back into the menu, not during the Sparring session!

Finally, Unlocking Costumes. 50GS
Below is a list of characters and how many Costumes they have...
Costumes are unlocked by completing Storymode for that character; or beating Time Trial on its default settings; or buying from Zacks shop for Online Credits

Given Time Trial on Default is impossible (win 2 of 3 matches.. impossible against that green clone thing ), I recommend buying Time Trial characters clothes from Zack.

Every character comes with 2 outfits available at the start.
Ayane 6
Bass 4
Bayman 3
Brad Wong 3
Christie 4
Eliot 4
Hayabusa 4
Hayate 4
Helena 5
Hitomi 8
Jann Lee 4
Kasumi 7
Kokoro 7
La Mariposa 5
Leifang 7
Tina 5
Zack 4

Ein - TT Only 4
Genfu - TT Only 4
Leon - TT Only 4
Spartan-458 - TT Only 7

Zack sells clothes for 5000 each, so thats 55,000 Credits required for the TT people. - It can be earnt by beating people online (which is easily boosted tbh)

There is also the survival mode (both Single & Tag), which has a sliding counter up to 100 people killed, personally, my max is around 25, good luck with attempting it!