Can I ride my bmx bike around for a good workout till I can buy a treadmill in a month or two? How many miles should I go? How intense?(heart rate?) I mean I got two jump ropes, but I know I need more...

Yeah it's fine, as long as it's comfortable. It's just a bike.

a BMX would be good, but if you ride a bike den you will have to bike for alot more miles then if u run.....and for miles....i'd say to start witt running about 2and work ur way up. i dont kno how many miles for bike tho...just double it.