DarkRP is a crime roleplaying gamemode which revolves around 3 main premises.

1 Do crime, make money.
2 Stop crime, make money.
3 Be civilian, make money.
EXTRA Beg for money.

In most servers, there are common ways to make money by crime, such as:
Drug manufacturing
Money printing
Kidnapping for ransom
Mugging
House raids
Paycheck

For Police there are:
House raids
Arresting criminals
Paycheck

There are multiple civilian jobs, such as:
Gun dealer
Pizza maker
Smoothie maker

The latter two of the civilian jobs are not as common, but you will see them in most popular servers.

And the final way of moneymaking:
Begging for money as a hobo

In most servers, criminals and civilians can own buildings to do activities in. Around the map, there are doors for sale, which you can deck out with GMOD props.

Servers have common rules for bases, like:
No crouch doors (raider should not have to crouch to proceed)
No one way props
No more than insert door number fading doors.

These rules differ from server to server.

Almost all servers have a "building" rule.
When you are building your base, you must put a textscreen sign outfront that says you are building. When building you can have no entities inside the base, such as money printers.

The server you play is all down to you, I will not tell you which servers to play, but I can tell you some of the best.

Elitelupus has his own DarkRP server averaging at about 80 players.

Fudgy has his own DarkRP server averaging at about 80 players.

ZARPGAMING is a classic, but averages at about 20 players.

There are 3 types of RP:
NSRP: Non-Serious Roleplay
SSRP: Semi-Serious Roleplay
SRP: Serious Roleplay

You'll find the SRP on Military RP servers, SSRP on most DarkRP servers, and rarely NSRP.

Your choice of RP is your choice, for you to decide.

Policing in DarkRP has it's benefits, and it's losses.
Criminal activity makes more money in the longrun, but policing is easier and you have a team.

Most servers have a criminal scanner that scans for criminal entities in a radius, and you can use this for a warrant on the base.

Ususally, Police Officers are tasked with arresting criminals, SWAT Officers are tasked with house raids, and the Mayor keeps control of the city.

The most common Server Rules are:

No discrimination
No NLR
No RDA
No RDM

RDM is the random killing of another player without a valid roleplay reason.
Some valid reasons are:
Following you after you give 3 warnings
Inside your base, aslong as you have a KOS (Kill on Sight) Inside sign.

RDA is random arresting, without a reason.
Some reasons are:
Guns out
Just mugged
Stole a car

NLR is New Life Rule, after you die you must forget your previous life for 3 minutes, you can't hold grudges past deaths, or go to where you died for 3 minutes. It doesn't matter how you died.

I hope this helped, if I forget anything, leave it in the comments.