Make sure you get an actual Doppler and not just a microphone pretending to be a Doppler eg angelsounds, especially if you want to try early on at 12 weeks. I use Dopplers at work and was lucky enough to pick up a used Huntleigh for £30 on eBay........you’ll likely see your midwife and obstetrician use the same type.
Also make sure you know what to listen for......placenta, umbilical cord, foetal heartbeat and your other pulses eg femoral artery all sound slightly different, but only cord and foetal heartbeat tell you that baby is ok. It is tricky at 12 weeks, even for someone who finds tiny pulses as part of their job like me. Several times I couldn’t find it after 20 mins trying and had to come back later on and try again. It is important not to worry about not finding it this early. With a full bladder is best in early pregnancy as it makes your uterus rise out of your pelvis a bit more, the ultrasound can’t hear through bone.