Hey there. Ok don't take this the wrong way, but when I read you have only been using Marth for 2 weeks I knew full well that you were going to have pure weak points. So I'll give ya some tips because you DO have potential to be a hot Marth.
Ok ....
1st : Learn to sweet spot Marth's Up B. That is so important. Cause if you don't, you'll get loose stocks so quick, not even funny. Especially against a better Marth.
2nd : You need to GRAB! Marth has such good range. Watch pure vids to see the various methods of which way to throw. That is what I did. Against Falcon, F-throw to down tilt works nicely. Yeah, start applying dash cancelled D-tilts. Its one of Marth's key approaches.
3. Learn to edge guard better. Down tilts, SHFFL fairs are sometimes good with this. Wavedash back to the edge, then use your bair. Oh and try running of the stage to attack at times, Marth can make it back quite nicely.
There is more...but I think you should focus on what I mentioned first. If you want me to say more I will, only if you ask, Of course I see other ppl telling you things I left out anyway.
Good luck.
Here...watch this one vid of me playing another Marth. Just look at how I edge guard. I do have flaws, of course. But its pretty good nonetheless.
Mainly for edge guard tips:
http://www.youtube.com/watch?v=XAtzfP3kqjM - The better one.
http://www.youtube.com/watch?v=d3493g5VPyM