You need to have awareness about the layers of the skin and how far a needle needs to go when you are tattooing someone. While there are seven layers of the skin, only two matter the most - the epidermis and the dermis (the outer layer and the layer behind it). The dermis is the layer which the tattoo needle needs to penetrate.
This is a very technical process and you need to know exactly just how deep the needle needs to go. Tattoo needles need to penetrate almost 1 to 2 mm into the skin. Any farther than this and it will cause pain to the person getting the tattoo. It can also cause blood poisoning because of the ink getting injected into the blood stream.
Before touching your client, a professional tattooist needs to set his needle. Remember the needle needs to stick out by 1/16 to 1/32 of an inch.
Never rush yourself before tattooing a person. It is best to first check the depth of the needle by puncturing a fruit and gauging the hole.
If there is unusually high bleeding, you must stop immediately and adjust your needle depth.
Search for a well known tattooist in or around your locality and ask him/her for tips and guidance. However, if you are competing with other tattoo artists in the area, you may want to get feedback from other sources. There are several tutorials and videos available online, which you can watch to improve your own skills.
However, the internet is no substitute for the real thing. If you find yourself struggling, you can always seek an apprenticeship with a veteran tattoo artist. Watching a professional at work can be very fruitful for you since most tattoo artists are possessive about tricks of the trade.