Should I diversify my Portfolio?

So I'm new to the world of investing. I hold about $20k of CSCO stock which I purchased through their ESPP with a 15% discount.

I don't hold any other stocks. I'm wondering if I should diversify a bit? I know the age old tale of “don't put all your eggs in one basket”, but I know next to nothing about which stocks are good buys or not for long and short term investments.

I'm only 25 but would like to purchase a house within the next 5 years and am willing to hold my shares for longer if needed.

Any advice or anywhere I can read up in here?

Thanks