First of all, buying lingerie for your sweetheart can be both an intimate and thoughtful present, but it depends on how you do it. If you are not careful, she might perceive your present, as merely you acting like a pervert.
So before you buy, consider why it is that you want to buy her lingerie? If it is just to see her in an especially slutty schoolgirl or nurse's costume, you are probably just going to get yourself into a huge heap of trouble!
The last thing you want is to end up sleeping alone or being in the doghouse because she took your present the wrong way. However, if you do get it right, your little surprise can be an especially sweet gesture of affection.
If you know her well, you will know what she likes. If she never wears a g-string, chances are she will still not want to wear one even if you buy it for her. Think of what kind of styles and patterns she usually likes. She will notice that you were considerate in your purchase, and it will help your present be something that gets used instead of just something that gets shoved in the back of her drawer.
Next, know your sizing. Even if you are absolutely clueless about the ins and outs of women's under things, there is an easy way to find out. Simply look for a bra she has left lying around or find one in her underwear drawer. Write down the sizes; this will make it very easy to find something that will fit her. Underwear sizes come in small, medium, and so on, or they come in number sizes. If you can, find a pair that has a number size, as that is more specific, and will guarantee a better fit.
Next, it will help if you know a little bit of basic lingerie terminology. Obviously, you are probably aware of bras and panties. Camisoles are like a bra with a built in sheer part that covers the midriff. Teddies are like little one piece's that cover the whole torso. Either of these are a great choice for a man to buy as a present because they suggest romance instead of kinkiness or lustiness.
Finally, a few last minute tips to help you avoid disaster. Do not buy anything that says 'control' or 'reducing,' or anything like that on the label. You can see how a present like that might be misinterpreted as you calling her fat.
In the same way, cleavage-enhancing bras could be interpreted as you calling her flat chested. And once again, avoid getting kinky items, because that generally does not say 'I love you.'
Then, finally, it is time for you to get to enjoy picking something out! Pick something that you know she will look ravishing in! Silky, lacy, or a colour you really like on her. Something like this will definitely send the message of how much you care for her and how beautiful you think she is.