Hello I'm new to the forum, it's my first topic and post ! 
I'm working on a project that need 3.3v from a 3.3v Li-po. I know that I need to use LDO type regulator but I don't know which to use.
In this project I realy want to keep the power consuption low and the circuit will be draining aproximately 50mA.
Thanks for you help and sory for my english, I'm french 
You do not need to use an LDO. If your project requires 3.3V, you can also use a step-up/step-down regulator like these: Pololu 3.3V Step-Up/Step-Down Voltage Regulator S7V8F3
Many projects do not require a specific voltage. The ATmega chip in the Arduino can run on any voltage between 1.8 and 5.5 V, although at lower CPU clock frequency with low voltages.
Thanks
The project operating at nominal 3.3V will almost certainly operate just fine at 3.7V. Check the datasheets for the ICs used.
However, you will need to disconnect the battery from the circuit (at least the Vcc line) to charge it.
Here a linear regulator perhaps makes more sense since the power loss is small and a
simple switcher might risk discharging the battery flat (dangerous). You can get chips
that act as regulators and LiPo management all in one, these are surface mount invariably
though.
You need a very low-drop-out regulator - this means reading the datasheets and finding
one with 0.3V or less (most are 0.5 to > 1.0V - marketers use terms like LDO very loosely)
Nearly all 3.3V devices are rated for 3.0 to 3.6V, and freshly charged LiPo batterys generate
4.2V or more, so certainly not compatible without a regulator. The regulator should have
automatic over-current and overheating shutdown - very important with LiPo as some
form of fuse / safety cutout is necessary due to the vast power such batteries can source.