Getting into first and second gear question?

I have 2003 5 speed ford mustang and its kind of hard to explain, but sometimes when starting into first gear the engine will kind of stall and pick up alternately jolting the body even when giving it good amount of gas, cant even floor it and it will still do it. Then, if i switch to second gear it will keep up with it until after a few seconds it will finally come out of it. Now i have been driving stick shift for only about a week now so im hoping its just something im doing wrong. But, for the first few days i wasnt having this problem. Any help me diagnose what the problem is? Thanks a ton.