When I try o start the engine but don`t do anything.

It is possible that you have a faulty alternator. You do get rare occasions where a faulty unit slips through the system and gets sold. Please read this all the way through as it give more than one way to check for where the problem could be.
How old is the battery? They are considered 'Old' once they are over 5 years old. If you know how to use a multimeter try this.
You'll need to check both the alternator and the battery. For the battery - With the engine OFF test the DC voltage of the battery. It should be somewhere between 12.6 and 13.2 volts. Anything lower than 10 volts and it could be a faulty battery. Even a fully charged battery can struggle to supply power if it is old or worn out.
For the alternator - Start the vehicle and retest the battery voltage. It should read somewhere between 13.7 and 14.7 volts. If it's noticeably lower (or higher) it is most likely the alternator.
If you don't have a multimeter you can try a basic test. Get a helper to stand in front of the vehicle. With the park brake On turn on your head lights THEN start the engine. Your helper is watching for a significant dimming in the headlights. The lights should dim as you crank the engine then brighten again. Next turn everything else on, including the stereo, A/C, courtesy light. This is to put the battery and alternator under heavy load. If the headlights dim noticeably It is possibly the alternator.
Most companies often have a guarantee or a warranty on the alternator so a replacement shouldn't cost you anything.

MAKE SURE ALL FUSES ARE OK AND MAKE SURE THE EARTH STRAP TO THE BODY OF THE CAR IS SECURE, THEN MAKE SURE THE BATTERY HAS A CHARGE COZ IF NOT IT WONT HAVE ENOUGH TO START THE CAR EVEN OFF THE SMALL BATTERY PACK,HAD THIS PROBLEM BEFORE.

The alarm system somehow got activated , this is usually because the battery becomes discharged or disconnected try doing this:
Remove the key from the ignition
disconnect the negative battery terminal
put the key back in and turn to ON
exit the truck and leave door open
reconnect the battery terminal
get in and leave door open, if the alarm horn stays off it should start.

I would check all of the main battery grounds first thing to make sure one is not connected good or rotted out.To do this you will need a multimeter set to OHMS one probe to the neg bat terminal and the red one to the engine block the reading should be vary close to 0 meaning vary little restance between the bat and the true ground the block ih the reading is high the connection or wire is bad The next check if its not a ground wire and we are asumeing that bat is good starter and altenator are connected corectly because the vehicle will start . Dis connect the neg cable from the bat. set the miltimeter to 20 volts DC. put the red probe on the battery and the black on the engine block must be a good ground point if you read voltage then there is a short in the system because the posivite side of the bat should have no path back to groundfrom the neg side of the bat At this point you need to start looking for things that don't work correctly that are electrical . it could be anything a sensor a motor a blower fan a light . Dose it drain the bat if it sits for a couple of days with out running These things are never easy to track down You can try pulling fuses one at a time and retesting the last test to see if the short goes away. But you have to do this one at a time it give you an idea what system is effected .Look around the vehicle see what isn't working correctly and disconnect the power wire from it but do the easyest thing first don't ripe out the dashboard if you can unplug a connector that is going to the suspected part and hopefuly with a little comon sence you can find the problem most of the time it dosen't cost a lot to fix but if you take it to an auto electriction it will cost you thousands to diagnoss and most of these guys just start replaceing parts until it works GOOD LUCK WITH IT

You recycle the old dead battery

Have no idea why you even put that back in

Common sense & your friend is correct

You don't disconnect anything on today's
vehicles, even with the key on

Put in a new battery,start the car, warm it up
& drive it for awhile & maybe you will get lucky
& it will complete some of the drive cycles & run some
monitors ,you know nothing about & it will be fine

If not, your mechanic friend should be about to stop
by with the bosses scan tool & help you out

Your problem may be one of the following:
First check or just replace the fuel filter. Usually located under the car in front of the rear axle, drivers side.
Next, the fuel pump may be worn out. Usually they get loud or stop working (running).
Next, check the fuel pressure regulator. Usually located on the front fuel rail, drivers side.
Is the vacuum line attached? Not leaking or rotted?
Also, do you smell a strong fuel odor when it won't start? May be one or more stuck-open injectors, or the coil pack is bad.
Checking these in the order listed will more than likely fix your problem.
The clicking noise indicates the battery can no longer run the starter, which also means the fuel pump won't run either.
It could be a fuel issue. How old is the fuel filter? Can you hear the pump run for a few seconds when you turn the key on? Try turning the key to ON without trying to start engine three times waiting for the pump to stop running each time. Then try to start the engine. If it starts, Its a form of Vapor lock caused by the extreme heat we have had lately, and a weak fuel pump.

If none of this works, try a new fuel filter first. From there have someone check the fuel pressure regulator and then the pump. If you are in need of a new pump, take my word for it and spend the money to get one from a Dealer, and not from your favorite parts store. Most parts store parts are made in China and they are JUNK.

You not telling us anything

Is the battery charged & fairly new?
Does the starter even turn the engine over?
Do you have spark & are only missing fuel?
Do you hear the fuel pump run

What engine
Who will be working on the car

How would we know what works & what doesn't
All we do is guess,but need some thing to do that

By removing the battery & the radiator the sensors sent signals to the ECM. You probably have to get the computer scanned & reset with a professional diagnostic tool. Take it to a reputable tech.

My 2007 GMC Yukon will not start in cold weather. When I turn the key over the lights/dash controls come on, but the engine does not turn over at all. Nothing. Put a new battery in and still does it. It is a random occurance. Would not start for two days this week so I had it towed to the dealership and the next morning it starts without the service dept doing anything??? Bring it home and two days later it does it again??? Any ideas?

i want to say that it sounds like the fuel solenoid closing . if the solenoid is going bad it will happen more frequently , if you have a cam sensor on that engine that would be the other possibilty .