IMHO, there are two possible first steps.
One is leave it in the ground and work on driving back the foliage. I see that you have interior branches, so one step might be to cut back to them if they have foliage. The hard part is to get a sense of what you will need. to 'get there'. The bonsai rule of thumb is that the height should be about 6x the trunk diameter. With pines, the fashion is even a lower multiple. So, measure the trunk caliper and multiply by 6 to get an idea of how low down your bonsai maybe should be.
The trunk is rather straight = boring and uninteresting. The way we deal with this is to cut the present trunk down to a where there is a branch (that point is a node) and then wire that branch up so that over the next few years it will form the next trunk section. Rinse and repeat and you have a zig-zagging trunk line, although trite, more interesting than a peg. Then you would dig it out and put it in a big pot. Over the course of several years you end up cramming it into the smallest pot you can (fit all the roots) or plant it on a slab. Then you enjoy it for years, refining it, keeping the foliage from running away from the trunk, developing replacement branches, cutting off old branches that have become too long = keeping pine bonsai. Of course, you are not required to make a zig-zag trunk. You could jin the upper part of the tree to resemble a tree damaged by lightning or draught. If you've seen things like this in a forest, use them as your inspiration.
The alternative, is to dig it out first. You will want to keep all the foliage you possibly can, as photosynthesis is fundamentally what drives root growth and recovery from root damage. In my experience this move from ground to pot is the riskiest. Too often the functioning roots are 'way over there' where it long ago found the water and air the roots needed. One may accidentally cut these off and kill the tree, yadda, yadda, yadda.
If you are really prepared to maybe lose the tree, dig it and put it in a pot this Aug/Sep. It will have new, highly productive foliage to drive its recovery. Give it all of next year to 'get its engine running' and then in 2022 (if it is growing vigorously) start making your bonsai. Then you will not have spent years creating something to then lose it trying to put it into a pot.
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