I live in the UK, so speak with a British accent, but there are often more American resources. Most movies and famous TV shows are american, so if you enjoy watching those and think you'll use them, you'll pick up an american accent easier, but if you watch a lot of British films/TV, British will probably be easier.
You'll also find it easier if you're using american lesson books and learning resources to learn an american accent and spelling, but if your using British resources, then you'll probably find it easier to use a British accent and spelling.
You'll also be influenced by your friends and teachers' accents when speaking English with them.
Hope this helps.