Nearly Illegible Prime Number Calculator This is my first reasonably large C# program. Considering my code looks like it was written in an esolang to me, I'm guessing I did something wrong at some point. My main priority is readability, although speed is also a plus.

using System;

public class PrimeSearcher {
    public static string Stringilate (int[] iterable) {
        string returnable="";
        foreach(int i in iterable){
            returnable+=i.ToString()+" ";
        };
        return returnable;
    }
    public static bool isPrime (int[] iterable,float target) {
        foreach(int i in iterable){
            if(target%i==0){
                return false;
            }
            if(i==0){ return true; }
            //reached the "end" (last discovered prime) of the array, hurray!
        }
        return true; //will only do anything for the last element
    }
    static public void Main () {
        int[] primes=new int[1000];
        float j=2; //float because I think it's the smallest datatype that can return a non-zero number when divided, correct me if I'm wrong
        while(primes[primes.Length-1]==0){
            if(isPrime(primes,j)){
                primes[Array.IndexOf(primes,0)]=(int)j;
            }
            j++;
        }
        Console.WriteLine(Stringilate(primes)); 
        //I don't *think* ToString worked when I tried it.
    }
}
By starting with j = 3 and incrementing j by 2 you could skip a lot of unneeded computations because it will skip even values.  
Instead of using a float I would go with int. Both are 32-Bit and you won't need the floating point values.  
You should let your variables have some space to breathe. 

Consider  

while(primes[primes.Length-1]==0){
    if(isPrime(primes,j)){
        primes[Array.IndexOf(primes,0)]=(int)j;
    }
    j++;
}  


versus  

while (primes[primes.Length - 1] == 0)
{
    if (isPrime(primes, j))
    {
        primes[Array.IndexOf(primes, 0)] = (int)j;
    }
    j += 2;
}  


where the later is much more readable. It uses the bracing style most C# developer use as well.  
Stringilate() could be improved as well by using string.Join() which would look like so  

public static string Stringilate(int[] iterable)
{
    return string.Join(" ", iterable);
}  


and as a side note, if you need to concatenate strings in a loop its much better to use a StringBuilder instead.
You should be consitent in your coding style (public static vs. static public).
You are using braces although they might be optional. Good choice !  
Instead of a concrete type you could use var instead