You should have watched or read a tutorial before trying to overclock. Only increase clock speeds in small increments and then check for stability.
Example:
1.) +25MHz core clock;
2.) test it using Heaven Benchmark, let it run all the way through.
If stable, repeat 1 and 2
If artifacts or crash occur see step 3
3.) -20MHz
now go back to step 2, if stable +5MHz and repeat step 2
..and do that until you find your max stable core clock speed.
Then repeat that process for memory. If you want to potentially save time, use increments of 50MHz and then decrease to 25MHz and then decrease again if you want.