Samsung Gravity 3 Won't Play Any Videos PLEASE HELP ME!?

I just bought a new phone from T-Mobile via contract renewal. It was a cheap and decent phone. Now I love to play videos on phones because I thonk its pretty cool to download videos from youtube and place them on my phone to watch like in a car or on the couch. Especially on a cute little screen. A few years ago I had a Samsung SGH-T639, which was a flip phone that supported many formats of video and never gav me any problems... supported pdf files, music, cool animation photos, photo editor and so on; I loved it! Anyways I was excited wen I got the phone and opened it up to try it out. When I got home I went straight to my computer to download videos and convert them onto my phone. AS I know the screen resolution is 240 by 360. I converted a video to 3gp 240 by 360 and put it on my phone. Unplugged the USB cable and went to videos on my phone. Pressed Ok and....BEEP! "Unsupported Screen Resolution". I was like "ok" and deleted it and redownloaded the video in 3gp 176 by 220. Dragged the video to my phone and unplugged the USB. Went to my videos on the menu and pressed OK..... BING!! "Unsupported Frame Rate" I was like "WTF?!" So then I tried downloading a video to mp4 format in both resolutions, put them on the phone and got the error messages again. I've tried so many softwares ; Free samsung video converter, Vio mobile video converter, media coder.net, Samsung PC Studio, youtube downloader. EVERYTHING! None of it worked. Ive tried tweaking with changing the video bitrate, audiorate, screen resolutions, formats, tried to download a jar media player for it to get around the error messages and doesnt even support jar apps. Googled many pages, tutorials on other ppl that had this problem, youtube, hours of research and NOTHING worked! Please help me! Please Anyone out there who can help me. I will be SOOO thankful!