Where I can know when it reaches the gain level of -6dbfs on the daw??
I do need to record with instrument or line signal???
Where is the bit depth??
There is no disadvantage of having the recording devices gain set at pretty close to 0, it just means that the signal is hot enough to not need any gain. Thats a good thing with relatively cheap preamps, its when they are working hard that they 'colour' the signal the most, but to be honest its only very a minor colouration and not of consequence to most home users.
-6Dbfs is simply 6Db below the top of the meters in your DAW or soundcard control panel (if it has one). Most DAW meters will go to +6 or +12, so just hit 6db below that max. In practice, I usually advocate hitting a lot lower than that, something like -18Dbfs, at 24 bit with modern A/D converters you are loosing nothing and it can make the whole mix process easier, especially if using lots of plugins or outboard gear.
24bit is normally a checkbox or option in the DAW, it will be different for different DAW's. In cubase its under 'project settings', in Logic its under 'preferences>audio' in pro tools its on the 'create new project' dialog box etc. You should always record at 24 bit unless you know of a very good reason not to.
You should be going into the instrument input, or into a DI then into the mic pre. It won't make a huge amount of difference going into a line in but the slightly wonky impedance matchup you are creating may effect the sound a little, making the highs a little more brittle etc. The instrument input SHOULD have a slightly more suitable input impedance. Going direct into a mic pre will most likely be FAR too hot and distort even at minimum settings, but its unlikely you've tried that, as it would require a jack to male xlr cable. A average DI box reduces gain suitably to go into a mic pre, though many active ones have outputs designed to go into Line Ins.
Instruments should go into an instrument in, or a di box.
Anything labeled line out should go to a line in.
Unless labelled otherwise DI boxes should go into mic pres.