Welcome Stephane :)
Sorry but there is no fool-proof 123 step process when it comes to selling online, there are various factors to consider like your location, your marketplace of choice the products you intend to sell.
Most seasoned sellers will always tell you to do your due diligence. Research and read all you can, digest the information and continue researching and reading before taking the plunge.
What to sell? Generally it's best to start selling something you are interested in because you are going to be spending a LOT of time thinking about it! Do you love fashion or movies? Perhaps you feel excited about the thought of new gadgets and technology? It's likely that there are plenty of other people who also share your interest - giving you an instant audience for your product. One idea is to jot down your interests and then make a list of products that are related to these interests (we recommend staying away from electronics to begin with as warranty issues can be complicated to manage).
You can also use our very own Link hidden: Login to view to help you find a profitable niche :)
Once you've thought of a couple of ideas, it's time to start researching suppliers. This is where SaleHoo comes in. Use the search box to look for the product(s) you want to sell. When you've found some suppliers who fit the bill, send them an email asking what the best possible price is that they can give you for the products you wish to buy. A small tip for you: if you buy 20+ units at a time, you should be able to negotiate a better deal. It's also a good idea to contact more than 1 supplier before you start buying to compare prices.
Hope this helps!
Btw, your English seems fine :)