Have a good nose around on here, you'll find a ton of stuff that will answer your questions.
You will come across enough credits throughout the game to help you get by. I think my level 50 ended on 2 mil and all purple 50 gear. I tried to do every side quest I could. I also went for scavenging, slicing, and treasure hunting. They brought me in a load of money mid level.
Don't worry about gear too much below level 15, you level it out way to fast. Save your cash for later on, you'll need it more then.
Don't sell your orange gear you can buy mods for it as you go.
Kill everything you can, also group when you can.
Always try to be at least 3 leveles higher than the plannet you are on (if you do all the side quests that shouldn't be a problem) that way you'll have no probs with mobs or bosses, it'll save you a ton on repair bills also.
You'll learn as you progress and by your mistakes.
Have fun.