Hi grunta and welcome.
yeah, as JDog says, are you just using keyboard or and you using a keyboard and mouse combo? The game was originally designed around using mouse to move and keyboard to rotate and it works really well once you adjust to it IMO.
A twin stick joypad will always be the ideal control method for this style of game for most people and playing with keyboard only isn't idea.
If you can use a mouse too then it should still be very playable. Although I prefer mouse to move and keys to rotate, I know a lot of people prefer using keys to move and mouse to aim and both set-ups are available in the options.
As for tactics well it can appear safer at the edges of the screen, however there's always a good chance you'll get hit by an asteroid entering the screen. Also it taking on a larger enemy then try backing away from it as you shoot as this will buy you a bit of extra time (particularly useful for those red asteroid belts or those cyan planets and moons)
As mentioned earlier though, if you are just using keyboard controls without a mouse then it will be a lot harder. Unfortunately the nature of the game means there's no way around that from a game design viewpoint that I can think of.