I would say ... A 125 2-stroke would be a good starting bike, just make sure you take it easy at first dont rush it ,,,,,get a feel for the bike. As for brand bike(honda, ktm, suzuki, kawasaki) Just choose your favorite color because they are all about equal specifications... Although the YZ 125 is a bit bigger than the others, KTM has a better engine but its handling isnt so good. RM and CR are good all around bikes and KX is also a good all around bike .
Also remember saftey first... Much better for a helmet to get cracked than your skull