Welcome Victor!
Victor Almeida said:
1. What scale is a 90mm figure?
No one answer to this unfortunately. With figures sold at nominal sizes - 54mm, 70mm, 90mm, 120mm etc. - there is a lot of variation in their actual size and their scale from maker to maker, even sometimes from piece to piece in one line.
90mm equates to 1/19 scale
if one assumes a given average height and measure from sole of foot to crown of head.
Victor Almeida said:
3. What tricks can you suggest to help me with the assembly and painting?
You've picked a pretty complex figure to begin with, so I'd recommend spending the time to familiarise yourself with the basics of cleanup and general prep and working through the steps slowly and methodically. Don't be afraid to do dry-runs on assembly before you commit to glue.
You can assemble a metal figure in about two minutes if you wanted to but it's well worth taking the time to do it right.
Search here and elsewhere online for further information on any of the various techniques you'll hear about - removing mould lines, pinning joints (especially important for heavy metal figures), what glues to use or soldering, holding parts during painting, whether to assemble first or paint in subassemblies, smoothing/polishing the casting, filling pitting or gaps and more.
Victor Almeida said:
4. What are the advisable tools and paints to start me up?
In terms of tools, different people use different things. The very simplest might be a single knife and a piece of fine sandpaper, but as you can see from the thread
Tools of the trade it can get a lot more complicated than that. You'll also need some two-part putty for filling, e.g. MagicSculp, Milliput, A+B, Apoxie Sculpt.
As far as paints go, you can use enamels, artists' acrylics, oil paints, alkyds, hobby 'acrylics' (Vallejo, Andrea, Reaper, P3) so there's a lot to pick from. If you don't have any brushes you'll need at least a couple of rounds and maybe a small flat; starting with synthetics might be a good idea rather than going with something more expensive this early.
BTW you also need to select a primer as it's important to prime first for best results. I recommend spray auto primers as they are inexpensive, good quality and very tough. Many hobby primers are multiple times the price.
Einion