How to put camera in automatic focus - Minolta Maxxum 400si 35mm SLR Camera

Not sure what could cause the white lines. Are you sure you did put the camera in automatic mode? if it is in any other position, it could be you force it to overexpose. In the manual are several causes, why the camera can have trouble focusing. too little contrast in the subject, is only one of the reasons.
Please check the manual for the settings of the camera.

With a digital camera, some things go different as we were used to with the film camera. The autofocus function, will try to prevent you from taking a picture when not focused.
With a digital camera, every picture should be ok. Than means, as soon as the camera "thinks" not enough light or not in focus, it won't release the shutter. Even when not in automatic, the camera wants to make sure it is in focus and has enough light. So try again when you have enough light. Then press the shutter release button half, till the camera is ready focusing and then press complete. If the camera still strikes perhaps it needs a repair centre?
Make sure you did place the camera in automatic, to exclude other possibilities that can cause "out of focus" or "wrong light"

I don't know how much experienced you are with a digital camera. Don't even know you have any experience with any camera. But the modern camera's are capable of taking sharp pictures, is used right.
But to let the camera do its work correct it should be setup correct and used correct.
Let me explain.
Most camera's have a possibility to make pictures in automatic. (dail on the green camera symbol) But still there is more to do.
Every camera has a point (or more points, that are used to focus the picture. You can use to put that point where you want the picture be in focus. Most of the time that would be the centre of the frame. But the camera can't focus in theoretical a blink of an eye. It needs some time, to find the focus by differences in contrast, or patterns. A DSLR sometimes can focus in less then a tenth of a second. Compact camera's could need up to one second, before they are in focus.

So try to setup the camera to automatic. Place the focus marks (as described on page 6 of the manual. Then press the shutter realise button half and look if the camera can focus. If it can't, release the button and try again. When the pictures is in focus, then you can press the button fully.
On page 33 of the manual, there are different modes of focus. be sure you use the mode you chose in the correct way.

link to the manual I refer to.

In automatic mode and automatic focus - The camera should focus onto the subject given that there's enough contrast between your subject and the background. If it not focusing at all even in broad day light conditions - You lens is faulty.
Also - Do you or your friend have another lens to test if the camera is faulty?

There may be a few things preventing the camera from taking a picture. I don't know the lens you are using but by chance does it have an auto/manual switch that has been switched to manual? Also make sure the lens is locked in position. The camera won't take a picture unless it has focus in the auto focus mode, this is normal. What are you trying to focus on? The camera auto focus system will search and fail to auto focus on surfaces that have little or nor contrast. For example glass, chrome, a blank wall, water, blue sky. Other conditions that auto focus fails is being to close to your subject, heavy fog, a dim lit subject, available light and night photography. Once the little green confirmation light comes on the camera has focus and will operate as it should.

Hi pablo_cornel, One simple question, is the camera in manual focus mode. On the right of the lens mount, as you look at the camera is a switch that reads AF and M. If that switch is set to M you are in manual focus mode. Also, does the 18-55 mm lens have a manual override switch?

Remove the lens and clean the contacts between the body and the lens with a pencil eraser. Make sure the camera is on Autofocus mode. If these don't solve the problem, take it to an authorized service center.

some cameras have a choice in menu as to monitor or continous autofocus-monitor or continous means camera always seeks a focus which hurts bat life but makes focus quicker-single means camera won't focus untill button pressed halfway-make sure you are not in macro mode-flower icon on screen-this will screw up focus except for close up work-good luck-check menu settings for your type of focus

I still don't have a digital SLR camera, but on the old Cannon film SLRs, the focusing motor is within the lense itself. The extender should have contacts to transmit the signals from the body to the lense. Visually inspect the extender, otherwise you'll be forced to go manual.

Check the rear optic on your lens. Then may be dust or debris preventing the focus to lock on your subject.