Im a female beginner for mx?

At the tracks I race in the US they have two cles for females, Girls 9-15 and Women 12 and up. There is no limitation on what bike you can ride. Which bike you choose really depends on how comfortable you are on a taller bike, how much more, if any, you think you will grow, and how competitive you think you will be at this point. The most logical choice for you is a 250F. You can ride a 250F at 5'5" but it will feel tall so it is just a matter of your comfort level on a bike that size. Your other option is the CRF150RB. You will be racing against girls on larger bikes but the 150R is pretty impressive and you may not be able to go any faster on the 250F than you could on the 150R. Unless you are very naturally gifted rider you won't be on the podium right away so if you feel more comfortable on a smaller bike for your first couple seasons get the 150R, but if you think you can get used to the size of the 250F go ahead and move to that now. Just go to your Honda dealer and sit on the CRF250R and CRF150R and then make your decision. I would also check with the tracks you plan to run to see what the rules for your cl are. The tracks I run are AMA sanctioned, but the ones in your area may not be and could have very different rules.