How do I make random structures

Hello, I want to make an infinite running game and i want to add some random structures. What i want to do is to create more of them and the Game put themselves together automatically.

System - Every x second ---- System create "object" on layer "0" on "random(xx,xx)

My bad, i ment this types of structures,

store the location of the last created object and with a x% propability move the next tile left, right or just ahead of the last one.

For example:

Create a random number from 1 to 100

1-20 put the next tile one step to the left (adjust width of the object to fit with the previous coordinates)

21-40 put the next tile one step to the right (adjust width of the object to fit with the previous coordinates)

41-100 just move ahead