The usual advice in these situations is that you don't start out by contributing 100% to stocks and then add bonds later on (unless you're building a bond tent, but that's something specific you do when you're approaching your retirement date).
Instead, you should decide on an asset allocation in advance, and every dollar you invest should follow that allocation. So for example, if you want to be 80% stock and 20% bonds, 80 cents of every dollar you contribute should go to your chosen stock fund and 20 cents to your chosen bond fund. If your portfolio drifts from those percentages, you contribute more to the fund that's under-allocated until you're back on target.
80/20 is what I use. From what I've read, it's almost as good for growth as 100% stocks, but with less volatility. But this depends a lot on you - on how aggressive you want to be and whether you have the temperament to stay calm in the face of market swings.
