Should a 15 year old girls knees lock and get sore as much as mine do?

Knees shouldn't hurt ever, but often times they do. If they only hurt after you fall then I would say you bruise them and twist them falling. However, if they always hurt then you might consider buying a knee brace. These can be very small and basic but still provide a good deal of support. Always practice RICE (rest, Ice, Compression, Elevation) after you have a particularly hard practice or notice that your knees start to swell. If they continue to hurt for a long time, consider visiting your doctor or a special knee doctor and maybe start a physical therapy program. Good luck!