1) How many movement points should I spend while entering a building via road ?
One movement point bacause my movement is done along a contiguous road ?
Two movement points bacause I am entering a building ?
Rulebook, page 46, for Building hexes: "Movement cost: 2 even if the hex contains a road". Also note that only squads can enter a Building hex, not any vehicles, even if the hex also contains a road.
2) Can I active a single squad with card "Go to the ground" in situation, when this squad is standing alone on the hex ? Or should I use this card only when I have two squads in one hex ?
First off, be sure you're using the Errata for this card, which is in the rulebook on page 47: “Action Phase: Place up to two concealed squad markers on two friendly squads located in the same hex. You may not choose squads that are adjacent to enemy units."
Since it says "up to two", you may use the card to place only one Concealed Squad marker. A squad cannot have more than one Concealed Squad marker, so technically the second "two" in the card text above is redundant and a bit confusing. But unless there is another ruling or errata I'm not aware of, you may use Go To Ground on a hex with a single squad in it. (You could also use it on a hex with three squads in it, picking up to two of them to become Concealed.)
Make sure the hex in question has some cover-providing terrain, or that you're not in Line of Sight to the enemy, otherwise the squads will immediately become unconcealed!
One other minor point: When you use Go To Ground, you do not "activate" the squads in question. You can Conceal them, and then activate them for an action later in the same round.