Bandhas!

"In Sanskrit mula means root, and thus Mula Bandha is the root lock. To find it, sit, stand, or even be in an asana, and if you are a man, contract the area between the anus and the testes. If you are a woman, contract the muscles at the bottom of the pelvic floor, behind the cervix." Jun 14, 2011 from this link

Mula Bhanda lift up on the pelvic floor           Image reference

To help you visualize where to Lift/Engage(I prefer to use the word lift rather than lock) for Mula Bhanda, superimpose the two above images on top of each other in your mind. To try an exercise take a nice deep breath in and engage and lift on the exhalation. Do about 5-10 of these breaths. Try this for several sessions until you feel comfortable with it. Remember, this is yoga so you are effectively engaging in a home practice when you try this exercise. 

Uddiyana Bhanda lift in and up around the naval area       image reference

If you feel like you can easily do the breath with mula bhanda you can move up to also engaging and lifting Uddiyana Bhanda on the exhalation, but release and make room for the breath as you inhale again. Continue to breath like this until it comes naturally enough to breath in this way as you do practice yoga postures add flow. Here is a link, but I still recommend doing engaging and lifting rather than locking at this stage.

The concept I am trying to get across is described here in Bo Forbes' Newsletter. Scroll down to The Asana Lab: The Breath and Bhanda Vinyasa.