Tile Collisions, how can I improve on this

[Yogen]

function onGround() {
    //grab the layer id
    var lay_id = layer_get_id("Tiles_1")
    //grab the tile set id
    var map_id = layer_tilemap_get_id(lay_id)
    
    
    var grounded = tilemap_get_at_pixel(map_id,x,bbox_bottom+vel_y)
    var onWall = tilemap_get_at_pixel(map_id,bbox_left+vel_x,y-20) - tilemap_get_at_pixel(map_id,bbox_right+vel_x,y-20)
    
    for (var i = -25; i < 25; i ++) {
        grounded = tilemap_get_at_pixel(map_id,x + i,bbox_bottom+vel_y)
    }
    
    show_debug_message(grounded)
    if grounded == 0 vel_y += global.grav
    else
    vel_y = 0
    
    if sides != 0 {
        dir = sides
        show_debug_message(momentum)   
    }
    
    if grounded != 0 && sides != 0 && onWall == 0 {
        momentum = approach(momentum,walk_speed,fric)
    } else if sides == 0 && grounded != 0 {
        momentum = approach(momentum,0,fric)
    }
    
    if onWall != 0 && wall_Leave < wall_Leave_Max wall_Leave ++
    
    if onWall != 0 && sign(onWall) != dir {
        momentum = 0
        vel_x = 0
        show_debug_message("you walk into a wall")
    } else if onWall != 0 && sign(onWall) == dir {
        onWall = false
        wall_Leave = 0
        momentum = approach(momentum,walk_speed,fric)
        show_debug_message("you leave wall")
    }
    
    if onWall != 0 show_debug_message("stuck to wall")
    
    vel_x = (momentum / mass)* dir
    
    if jump state = jumping; is_jumping = false
}

function inputs() {
    sides = keyboard_check(vk_right) - keyboard_check(vk_left)
        //set if jumping or not
    jump = keyboard_check_pressed(vk_space)
    teleporting = keyboard_check_pressed(vk_shift)
}

Hi, I wrote this from scratch using some formulas for kinematics. any advices without using a tutorials?