How can I teleport players to start if they touch these blocks? [duplicate]

Try this:

execute @a ~ ~ ~ detect ~ ~-1 ~ minecraft:grass -1 tp @a[c=1] <x> <y> <z>

That checks if the player is on top of a grass block. Just change the relative coordinates (the ~ ~-1 ~) in order to check above, right, etc. And, obviously, change minecraft:grass to whatever block you want to check. If you have issues with the relative coords, check them out here.

The OnGround byte tag specifies whether or not the player is on the ground. You can assign a score based on its value and target the player based on that score:

/scoreboard objectives add OnGround dummy

/scoreboard players set @a OnGround 1 {OnGround:1b}
/scoreboard players set @a OnGround 0 {OnGround:0b}

/tp @a[score_OnGround_min=1] X Y Z

Another method is to detect whether there is air at the player's current feet position, but keep in mind that this method is flawed when it comes to players standing on non-full blocks such as slabs:

/scoreboard players set @a OnGround 1
/execute @a ~ ~ ~ detect ~ ~ ~ minecraft:air 0 scoreboard players set @a[c=1] OnGround 0

For 15w47c+, you can use the "tags" feature to avoid having to use scoreboard objectives:

/scoreboard players tag @a[tag=!OnGround] add OnGround {OnGround:1b}
/scoreboard players tag @a[tag=OnGround] remove OnGround {OnGround:0b}

/tp @a[tag=OnGround] X Y Z