How to put gravity

Can someone say me how to put gravity in a game pls?

You said "pls" because it's shorter than "please", so I'm going to say "No" because it's shorter than "Yes". If you want an answer that is actually useful, you'll have to post your code. Do you have a system for storing which sprites (or BG tiles) are for the ground vs which ones are actual objects that need gravity applied? Generally speaking, for each sprite that you want to apply gravity to, you'll decrease it's y-coordinate by some amount (something exponential if you want it to be realistic) and check for collision with the static terrain. If it's collided, do something to signify that it doesn't need gravity applied. We really do need your code.

BED3CE8E (probably won't be up forever, get it while you can). Below is a sample program showing off gravity. Feel free to ask questions about parts you may not understand.

OPTION STRICT

'USED TO SHOW THE PATH OF FALLING/JUMPING
VAR MAX_POINTS = 50
VAR POINT_X[MAX_POINTS]
VAR POINT_Y[MAX_POINTS]
VAR POINT_COUNT = 0

'CONSTANTS
VAR GRAVITY = 9.8 'METERS PER SECOND
VAR PIXELS_PER_METER = 16
VAR MS_PER_SECOND = 1000
VAR MIN_TIME = 16 '1/60 SECOND
VAR BTN_DOWN = 0
VAR SCREEN_W = 400, SCREEN_H = 240
VAR SPRITE_W = 16, SPRITE_H = 16
VAR TILE_SIZE = 16
VAR BG_SKY = 639
VAR BG_GROUND = 610
VAR BG_W = SCREEN_W / TILE_SIZE 
VAR BG_H = SCREEN_H / TILE_SIZE
VAR CAT_ID = 856
VAR CAT_SPEED = 12 'METERS PER SECOND
VAR CAT_JUMP = -25 'JUMP POWER

'GLOBAL VARIABLES
VAR BTN 'BUTTON STATE
VAR CAT 'CAT SPRITE
VAR GAME_OVER = FALSE 'GAME LOOP EXIT FLAG
VAR MAY_JUMP = TRUE 'ALLOW JUMP
VAR I 'LOOP ITERATION
VAR X, Y, DX, DY, Y1, Y2 'POSITION
VAR T1, T2, DT 'TIME

SETUP_WORLD
SPCLR 'CLEAR ALL SPRITES
GENERATE_CAT OUT CAT, X, Y

T2 = MILLISEC 'SEED T2 FOR DELTA TIME CALC

WHILE GAME_OVER == FALSE
 'CALCULATE TIME BETWEEN FRAMES
 T1 = T2
 T2 = MILLISEC
 DT = MAX(MIN_TIME, T2 - T1)
 
 'RESET X SPEED
 DX = 0
 
 'READ BUTTON STATE
 BTN = BUTTON(BTN_DOWN)
 
 'HANDLE BUTTONS
 IF (BTN AND #LEFT) != 0 THEN
  DX = -1 * ((CAT_SPEED * DT * PIXELS_PER_METER) / MS_PER_SECOND)
 ENDIF
 
 IF (BTN AND #RIGHT) != 0 THEN
  DX = ((CAT_SPEED * DT * PIXELS_PER_METER) / MS_PER_SECOND)
 ENDIF
 
 IF (BTN AND #A) != 0 THEN
  IF DY == 0 AND MAY_JUMP THEN 'NOT FALLING OR JUMPING
   DY = CAT_JUMP
   MAY_JUMP = FALSE
  ENDIF
 ELSE
  MAY_JUMP = TRUE
 ENDIF
 
 IF (BTN AND #X) != 0 THEN
  GAME_OVER = TRUE
 ENDIF
 
 'UPDATE LOCATION
 X = MAX(0, MIN(SCREEN_W - SPRITE_W, X + DX))
 DY = DY + ((GRAVITY * DT * PIXELS_PER_METER) / MS_PER_SECOND)
 
 'DON'T WANT TO PASS THROUGH GROUND CHECK EACH PIXEL BECAUSE LAZY
 Y2 = MAX(0, MIN(SCREEN_H - SPRITE_H, Y + DY))
 IF DY >= 0 THEN
  FOR Y1 = Y TO Y2
   IF IS_ON_GROUND(X,Y1) THEN
    Y1 = Y1 - ((Y1 + 1) MOD TILE_SIZE)
    DY = 0
    BREAK
   ENDIF
  NEXT Y1
  Y = Y1
 ELSE
  Y = Y2
 ENDIF
 
 'UPDATE POINT TRAIL
 ADD_POINT X + SPRITE_W/2, Y + SPRITE_H / 2
 
 'UPDATE SCREEN
 'MOVE CAT
 SPOFS CAT, X, Y
 'DRAW POINT TRAIL
 GCLS
 FOR I = 0 TO POINT_COUNT - 2
  GLINE POINT_X[I], POINT_Y[I], POINT_X[I + 1], POINT_Y[I + 1], #WHITE
 NEXT I
 FOR I = 0 TO POINT_COUNT - 1
  GPSET POINT_X[I], POINT_Y[I], #BLACK
 NEXT I
 VSYNC
WEND
LOCATE 0, 1
PRINT "GOOD BYE"
COLOR #TWHITE, 0
END

DEF ADD_POINT X, Y
 VAR I
 
 IF POINT_COUNT < MAX_POINTS THEN
  POINT_X[POINT_COUNT] = X
  POINT_Y[POINT_COUNT] = Y
  POINT_COUNT = POINT_COUNT + 1
 ELSE
  FOR I = 0 TO MAX_POINTS - 2
   POINT_X[I] = POINT_X[I + 1]
   POINT_Y[I] = POINT_Y[I + 1]
  NEXT I
  POINT_X[MAX_POINTS - 1] = X
  POINT_Y[MAX_POINTS - 1] = Y
 ENDIF
END

DEF GENERATE_CAT OUT ID, X, Y
 SPSET CAT_ID OUT ID
 X = (SCREEN_W - SPRITE_W) / 2
 Y = 0
 SPOFS ID, X, Y
 SPSHOW ID
END

DEF IS_ON_GROUND(X, Y)
 VAR CELL_X = FLOOR((X + (SPRITE_W / 2)) / TILE_SIZE)
 VAR CELL_Y = FLOOR((Y + SPRITE_H + 1) / TILE_SIZE)
 VAR TILE = BGGET(0, CELL_X, CELL_Y)
 RETURN TILE == BG_GROUND
END

DEF SETUP_WORLD
 CLS
 GCLS
 GPRIO -255
 
 BGSCREEN 0, BG_W, BG_H
 BGFILL 0, 0, 0, BG_W - 1, BG_H - 2, BG_SKY
 BGFILL 0, 0, BG_H - 1, BG_W - 1, BG_H - 1, BG_GROUND
 BGFILL 0, BG_W / 2, BG_H / 2, (BG_W / 2) + 8, BG_H / 2, BG_GROUND
 
 LOCATE 0, 0
 COLOR #TWHITE, #TBLACK
 PRINT " JUMP    LEFT AND RIGHT    EXIT";
END