The penis goes into the vagina very easily, although there are many stages so it goes in "right." When the penis goes into the vagina it must go in far enough that - if you are a virgin - it will break a small layer of tissue covering up the vagina known as the hymen. If the woman is not a virgin, the penis will slide in without any difficulties.
The vagina is actually the name of the opening (or canal) in the woman's body, a natural orifice like the mouth or anus. During sexual intercourse, the male's penis is inserted into the vagina. If the penis releases ejaculatory fluids, it can result in pregnancy (the woman can conceive a baby).
The ejaculated fluid is called semen, and contains the male sex cells, or sperm. If the sperm cells enter the vagina, they will attempt to fertilize an egg cell from the ovaries, which will become a fetus and eventually a baby.
Note: To avoid injury or discomfort, be sure that the vagina is lubricated before intercourse. Normally bodily contact is sufficient to begin this process. If natural fluids do not provide a comfortable insertion, there are artificial lubricants (such as KY jelly).
Using a Condom:
It really depends on if you're using a condom, or if you're not and she is using birth control. Condoms are usually lubricated, maybe not enough, but it helps a lot and they're smoother. It also helps protect against STDs, and contains the ejaculate (which minimizes the mess).
The vagina itself is where the penis enters, between the labia (the outer lips or flaps).
From top to bottom - "top" being nearest the belly button - there's the clitoris covered by the clitoral hood, the urethral opening (where urination occurs), and then the vaginal opening.
The most important thing is to first be sure that the female is receptive and has given permission for the penis to be inserted into her vagina. That is step number one!
1. Begin by ensuring the vagina is properly lubricated, either through oral or manual stimulation or a store bought lubricant.
2. Inserting the penis into a dry vagina may cause the female a painful and unpleasant experience.
3. Proceed slowly by using the head of the penis to feel for the opening of the vagina. Do not stab at it. Be gentle.
4. Place the head of the penis into the vagina and slowly lean in to finish the insertion of the penis.
The penis is now inserted and sexual intercourse can proceed.
A Guys Perspective:
The best way to approach this is to make sure the girl is adequately aroused. You do this through foreplay (sexual touching and caressing before intercourse.)
You can also use vaginal lubricants such as Astroglide, Surgilube, or Slippery Stuff.
The most common position for insertion is the Missionary position, this is when the woman is on the bottom and the two face each other.
Don't rush, the first time needs to be enjoyable for you both. Now that you are sure there has been adequate foreplay and you are both ready for insertion position the tip of your penis at the opening of her vagina and gently push, the wetness and gentle pressure will guide you inside her. It is perfectly ok to look at what you are doing, or to use your hand to help, her wetness and the position of her hips make it a surprisingly easy target to hit. Go slowly, a little at a time especially if it is her first time too as it can be quite painful. Gradually work your way up to full insertion and thrusting movements once you are able to move in and out with ease and comfort.
Generally, If the penis is hard enough and the vagina is wet enough you can just slide it on in.
Factors that may prevent insertion would include; the penis is not erect enough, the vagina is not wet enough or lubricated, the vaginal opening is too small for insertion ( this would typically happen during the first time or few times a female had sex if her hymen has not been broken or and or the vagina has not been stretched), penis is too large (the vagina has a surprising amount of elasticity and can generally be conditioned during sex or foreplay over time to cure any discomfort) Any discomfort to the woman or man can prevent insertion. Discomfort could come from a number of variables including inexperience, injury, too much sex or foreplay, STD's (sexually transmitted diseases) etc.