Hello, I have an Acer palmatum from a nursery and I want to start styling it. Because it's my first experience, I am open to any advice.

 

This is a grafted plant, and the graft is very visible. It is a pity, but it will not be a good bonsai, just something for you to practise on. I would allow the plant to grow for at least this year strongly, then next year possibly, or the following year you can start to style. If you must you could do a bit of light pruning this year, but I would not. I might drop into a larger pot if it has lots of roots, if not I would leave it.

Some grafts actually heals over to a non visible scar by time. Not all, but a few. I think this one have a  chanse. If not it is always possible to air layer at anytime to get rid of it.

For this type of material and a beginner I would advice to make broom style bonsai. Thicken the trunk in ground for a few years. Big chop to bare stump. Thicken primary branches for a few years until proportion with the trunk. Cut back. One or two years later when recovered, lift into training pot and continue growing secondary, tertiary, quariary branches the same way. Grow until in proportion and cut back hard. When the basic structure is there, repot into final pot and start refining.

Thanks for your answers. In fact the scar of the graft is quite evident.

Do you think I should already air layer it?

Huhh!! That is an ugly graft. My take on it is to walk away and never buy it. You must be more picky when shosing materials to work with.

Now, when you already have the tree, airlayer is the solution. If not anything else gained from it, at least you get some experience.

That graft and airlayer will delay the creation of a nice bonsai by 2-3 years.  

Thanks, I will start with the airlayer.
Unfortunately I didn't see it before because I bought online from a famous nursery. At least it was cheap...

Understood. At least now you understand that it is not possible to shop good starter materials online. No matter how famous a nurcery is, the suitable bonsai materials is 1% or less of what they have in stock. Two dimensional photos cannot tell the whole truth. I got fooled myself from your first photo. One needs to look at the tree from all angles and evaluate the material IRL.

I search all my nearby nurceries several times per year. That is 30+ visits. But only find one or two good candidates each year. It pays off to be picky in the long run. There are several guides on youtube on how to pick a good starter material. It can be very educational to see how other people shose. But in the end it is your own preferences and what future you see yourself in a material that counts.

I think the airlayer is going well, because I see the first roots. At the beginning of September I am planning to cut it.
Then I need some advices for the next steps to make the tree growing and the trunk thicken:

1. When I cut the airlayer and repot it, should I also cut a bit the brenchis to reduce the foliage?
2. Which kind of soil should I use to make the tree grow?
3. Should I use a deep or flat-ish pot? Or maybe a colander?

Nobody can help me with my questions? 

1: some people do cut branches and repot at the same time. Others say not to do two big work at the same time..I am from the second shool.

2: if you ask 10 bonsai artists about what is the best soil, you will get 12 different answers. It is up to you to find out what works best for you in your care and climate. 

3: It depends on your vision for the tree. But sinze you asked about growth in question 2, I asume you want the tree to thicken up? In that case a shallow pot is not an option.