dataGridView cells formatting

[Mathend]

Hi, guys. Sorry to bother you, but I need a little help.
I have a very simple dataGridView control (4 columns, 3 of them read-only). I need:

1) to keep users from entering characters other than 0 and natural numbers (1 to 20) on column 3. Currently, it's formatted as numeric, zero decimals, but accepts anything, including spaces.

2) to have row 0, column 3 as the selected cell when the DGV first opens, so the user can start entering information right away. So far, these instructions:

this->dataGridView1->ClearSelection();
this->dataGridView1->Rows[0]->Cells[3]->Selected = true;

are not working.
I'd appreciate your help. Thank you very much!