To the naked eye most BMX forks look the same. But there's subtle differences that may make one pair better for you than another, knowing these differences can help. Most forks have straight legs and dropouts at the front where you insert you wheel, the distance from the fork legs to where the wheel sits is called an an Offset. Historically BMX forks have an offset of about 33mm, but there are forks available with much shorter offset these days. The shorter the offset the easier it is to balance and move around on the front wheel.
Most fork Dropouts these days are 10mm or 3/8th in imperial measurements. You should buy in accordance to what size your front wheel is. Again most front hubs these days are 10mm / 3/8th but older style front wheels may be 14mm. Its easy to find out what yours is by measuring the edge of you axle with a ruler or tape measure.
Here at FoundationBMX we have nearly 20 years worth of experience with BMX Bikes. So if you can't make your mind up, or want some un-biased advice, send us an email at email@example.com.