Practice by remaking real life objects. You won't be detailed at first, but eventually you'll get more creative and more detailed.
Seriously though. Remaking stuff is a good way to ease yourself into building.
Also, don't go big: go small. Work on things you plan to complete, and this way you'll avoid a lot of unfinished projects. Finished projects will motivate you because you'll be able to tell yourself that the work you put into it paid off.
If you do decide to go big though, take things ONE STEP AT A TIME.
Do a little each day or each week.
Even if it's just placing a few bricks, it's STILL PROGRESS.
Doing a lot all at once can burn you out.
Building takes a lot of work and a lot of discipline, but don't be discouraged.
The only way you can learn something is if you try.