If you are falling through the floor it probably means that you've placed the info_playerstart too close to the floor (or inside it). To make a brush untouchable select it, right-click -> To Entity then select func_illusionary from the dropdown list.
These are basic HL editing techniques and should be directed to the Valve ERC (see the mapping forum FAQ for the address)
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These are basic HL editing techniques and should be directed to the Valve ERC (see the mapping forum FAQ for the address)