Your background, mainly whether you are smart, tough or charming or a combination of theese will give you some help learning skills that are "equal". I.e. if you are tough, it will be slightly easier for you to learn the tough skills. (It will cost you less skill points).
The clothes and items that are favorable for you are according to the skills you got. If you have charming skills, items/clothes with positive charm will aid you when you when you use theese skills, while the tough adjustment of your clothes don't mean something. - and the other way round.
Short and simple put:
If you go for Sweet Talking/Rule Bending, clothes and items with positive charm is what you will want to look for/keep.
If you go for Interrogation/Intimidation, clothes and items with positive tough is what you will want to look for/keep.
- and it's wise to go for EITHER Sweet Talking/Rule Bending OR Interrogation/Intimidation, not both.
The smart adjustment of your items/clothes aid you with the smart skills, that you mainly use to try to analyze the evidences of a case. Be sure to take the basic hair and basic thread skill, because then you will find all the evidences. The smart adjustment are working with the advanced smart skills and will help you to find what kind of hand/foot/hair/thread evidence you got (not finding the basic evidences).
There are a lot of helpful guides out there. I am sure someone else will be able to response to this thread with links to some of the guides, or send you a PM telling you ;-)
Happy Sleuthing :-)
Huglover has done a very nice brief intro about equipment up there ^
If you're looking for more, there is a Newbie FAQ at the top of the Newbie Board which contains some information about this. Also, if you dig through the Newbie Board you will find numerous posts about almost anything and everything you could possible want to know about equipment. Try the Sleuth Community Sites (link from City Hall) for yet another perspective on this varied and perplexing subject.
And finally good luck, and happy sleuthing.