Java Memory Problems

I have my own server and more and more often I have been getting this message in my terminal that I'm running the server on:

19:00:05 [SEVERE] java.lang.OutOfMemoryError: Java heap space
19:00:06 [SEVERE] at net.minecraft.server.NibbleArray.<init>(SourceFile:9)
19:00:06 [SEVERE] at net.minecraft.server.ChunkSection.<init>(SourceFile:25)
19:00:06 [SEVERE] at net.minecraft.server.ChunkRegionLoader.a(ChunkRegionLoader.java:252)
19:00:06 [SEVERE] at net.minecraft.server.ChunkRegionLoader.a(ChunkRegionLoader.java:63)
19:00:06 [SEVERE] at net.minecraft.server.ChunkRegionLoader.a(ChunkRegionLoader.java:52)
19:00:06 [SEVERE] at net.minecraft.server.ChunkProviderServer.loadChunk(ChunkProviderServer.java:135)
19:00:07 [SEVERE] at net.minecraft.server.ChunkProviderServer.getChunkAt(ChunkProviderServer.java:78)
19:00:07 [SEVERE] at net.minecraft.server.PlayerInstance.<init>(PlayerInstance.java:26)
19:00:07 [SEVERE] at net.minecraft.server.PlayerManager.a(PlayerManager.java:53)
19:00:07 [SEVERE] at net.minecraft.server.PlayerManager.movePlayer(PlayerManager.java:149)
19:00:07 [SEVERE] at net.minecraft.server.ServerConfigurationManager.d(ServerConfigurationManager.java:168)
19:00:08 [SEVERE] at net.minecraft.server.NetServerHandler.a(NetServerHandler.java:411)
19:00:08 [SEVERE] at net.minecraft.server.Packet10Flying.handle(SourceFile:126)
19:00:08 [SEVERE] at net.minecraft.server.NetworkManager.b(NetworkManager.java:229)
19:00:08 [SEVERE] at net.minecraft.server.NetServerHandler.a(NetServerHandler.java:112)
19:00:08 [SEVERE] at net.minecraft.server.NetworkListenThread.a(NetworkListenThread.java:78)
19:00:08 [SEVERE] at net.minecraft.server.MinecraftServer.w(MinecraftServer.java:554)
19:00:08 [SEVERE] at net.minecraft.server.MinecraftServer.run(MinecraftServer.java:452)
19:00:08 [SEVERE] at net.minecraft.server.ThreadServerApplication.run(SourceFile:490)
19:00:08 [SEVERE] Unexpected exception
java.lang.OutOfMemoryError: Java heap space
at net.minecraft.server.NibbleArray.<init>(SourceFile:9)
at net.minecraft.server.ChunkSection.<init>(SourceFile:25)
at net.minecraft.server.ChunkRegionLoader.a(ChunkRegionLoader.java:252)
at net.minecraft.server.ChunkRegionLoader.a(ChunkRegionLoader.java:63)
at net.minecraft.server.ChunkRegionLoader.a(ChunkRegionLoader.java:52)
at net.minecraft.server.ChunkProviderServer.loadChunk(ChunkProviderServer.java:135)
at net.minecraft.server.ChunkProviderServer.getChunkAt(ChunkProviderServer.java:78)
at net.minecraft.server.PlayerInstance.<init>(PlayerInstance.java:26)
at net.minecraft.server.PlayerManager.a(PlayerManager.java:53)
at net.minecraft.server.PlayerManager.movePlayer(PlayerManager.java:149)
at net.minecraft.server.ServerConfigurationManager.d(ServerConfigurationManager.java:168)
at net.minecraft.server.NetServerHandler.a(NetServerHandler.java:411)
at net.minecraft.server.Packet10Flying.handle(SourceFile:126)
at net.minecraft.server.NetworkManager.b(NetworkManager.java:229)
at net.minecraft.server.NetServerHandler.a(NetServerHandler.java:112)
at net.minecraft.server.NetworkListenThread.a(NetworkListenThread.java:78)
at net.minecraft.server.MinecraftServer.w(MinecraftServer.java:554)
at net.minecraft.server.MinecraftServer.run(MinecraftServer.java:452)
at net.minecraft.server.ThreadServerApplication.run(SourceFile:490)
>
So how do I fix this? Do I need more memory or can I do this by just changing the settings in java? (I'm on a mac)