If you wish to assemble a computer yourself, you should probably start reading a bit around, getting an idea about what components are needed and how do they fit together. Which are most important to what you want to do with your computer. Once you have done that, look at the graphs and reviews here at toms to get a good idea how best to spend your budget.
Then you can post your ideas on this forum and people can give you feedback on it.
That is what I did when I was new to this.
If you do not wish to spend time doing so, i suggest looking at the a prebuilt computer instead (there is a forum for that too)