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Hi Marcelo, welcome to IIS!
The Canon 400D is a great camera, and a great astro camera. The low noise and high sensitivity make it perfect for astro use.
The stock lenses are "ok" to start with. They're not great, and have some imperfections but they'll be enough to get you started. The 18-55 is worst at the 18mm end especially when it's not stopped down. The corners will show a lot of coma (stretched stars) and there's a bit of CA (Chromatic Abberation) on the bright stars. The 75-300mm is a great zoom lens for long focal length stuff, but may not be suitable for what you want to do.. let me explain further.
You've got a fixed tripod, so you'll be looking at a fixed piece of sky. As the Earth turns, the stars move. The longer the focal length, the quicker the apparent motion of the movement of the stars.
At 18mm on a fixed tripod, you'll be able to do a long exposure of around 15-20 seconds without getting star trails, but any longer than that and you'll start getting trailing. At longer focal lengths, for example the 75mm lens, you may only get 5 seconds exposure before you start getting trailing. It also depends which area of the sky you're pointed at. Best to just do some trial and error.
Regarding settings,
- Set the lens to the desired focal length (for eg: around 20mm on the 18-55).
- Set the lens to autofocus and point it at a bright star and hold the shutter halfway until it finds focus. Then change to manual focus, taking care not to change the zoom or focus. If you can't lock on autofocus, you'll have to try and manually focus it - this can be difficult and may need a series of test exposures to see if you're focused.
- Set the camera to Manual mode
- Change the exposure to f/4 or so - 1 or 2 stops down from wide open
- Change the shutter to 20s exposure.
- Change the ISO to 800.
- Leave White Balance on AUTO.
- Set the Quality to RAW if you normally shoot in RAW. If you're unsure what that means or how to process RAW, set it to Highest Quality before RAW.
- Click away!
Once you've got a bit of practise, you can do some more advanced settings like In-Camera Noise Reduction ON (in the custom functions) and also mirror lockup ON (again in the custom functions).
Do you have a remote?
If you do, set the camera to remote and then you can do your own timed exposures, from 15s to a minute or more, and you can see the effect of the star trails at different focal lengths and different exposure times.
I hope that helps, keep asking questions and make sure you post your results so we can help further!
Where is Carrum Downs? What's the sky like? Dark or light polluted?
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