Let’s start with 2) first.
MIDI sounds tinny for many reasons. First, many MIDIs are recorded simply as notes, and note values, without discreet changes in volume. Thereby, all the music is performed at the same dynamic.
But not all. Some do have that data. A lot of MIDIs are also step-sequenced rather than recorded live, also giving you a robotic and tinny feel.
Third, string instruments, brass, and the such are difficult to model digitally. The problem is there is SO much variation based on attack, volume, technique, etc. When a cellist plays an ff passage, the overtones are quite different than from a pp passage. Depending on your MIDI playback instrument, a lot of this stuff may not be modeled. The most “accurate” synthesizers have separate samples, nearly spaced, for notes played at a variety of dynamic levels and attacks. Your sound card, however, will not have nearly as much sample data to work with. It might just have a single sample of a cello, played at minor third intervals (with the remaining notes interpolated), at full volume. An envelope will be programmed with amplitude (volume) and frequency filters which imitates how an instrument sounds, based on how loudly its played. Louder -> more overtones, less filtering. Softer -> less overtones, more filtering.
MIDIs can sound very very good. All a MIDI is is performance data. You can record Elton John playing on a Roland digital piano as a MIDI and when played back on the same instrument or a MIDI instrument with a good piano sample library, it will sound just like the original.
But for most sound cards, the MIDI samples leave much to be desired.
As for 1)start here.
Most important to get realistic sounding drum patterns onto your computer is to play them yourself on a touch-sensitive pad or keyboard. If you want to do techno or electronic sounding drums, step sequencing (inputting them individually, one note value at a time) is okay. But if you want something with a groove, you’ll need to play them in yourself. (Or at least parts of the rhythm. Depending on what you’re going for, it might be okay to step-sequence the kick or the snare, but the high-hat you definitely want to do yourself.)