I want to use clock oscillator...

to slow down your clock:

process(n_clr, clk)

begin

if(n_clr='0') then

cont_reg <= ( others => '0' );

elsif(clk'event and clk='1') then

cont_reg <= cont_prox;

end if;

end process;

cont_prox <= cont_reg + 1 when ( cont_reg < 50000000 and cont_on='1' ) else

( others => '0' ) when ( cont_reg = 50000000 and cont_on='1' ) else

cont_reg;

tick_1s = '1' when ( cont_reg = 50000000 ) else

'0';

to start your clock with a button make a state machine:

process...

if(n_clr='0') then

est_reg = ini;

elsif(clk....

est_reg <= est_prox;

end....

process(...

est_prox <= est_reg;

cont_on <= '0';

case est_reg is

when ini =>

if (button ='0' ) then

est_prox <= contando;

end if;

when contando =>

cont_on <='1';

...