1 - Try new champs when they are available in the Free week. All champions can be good if you know how to use them so just choose a champion you think it's interesting and play it. This is a game, play to have fun and improve.
2 - You have to evaluate when you can roam, that's part of what makes a good midlaner/LOL player. Some tips are...
*Don't roam when there are minions your tower (you might not get anything from your roam and will lose CS)
*Pay attention to the enemy mid laner, he might follow you and turn the gank around or he can gank your other lane and get a kill there.
*Don't force roams non-stop, it might become too predictable and their team can intercept you on your way.
3 - What champion are you are using? You should pick a spell that will help the champion you are playing. Ignite, barrier, heal, cleanse, teleport and even ghost are viable choices.
4 - Who said it is? It's important in high level play, not in bronze. Forget about counter picks, just picks the champions you are good with and learn the match ups. It's much better to learn the champion match ups so even if you are playing against a champion that is good against yours you can still play well.
Some tips...
Assassins are good for 1v1, but they are not good for team fights (except Katarina) and mages usually are good for team fights because of their utility or AOE damage, but have trouble 1v1 assassin. So if you are playing Vel'koz against Zed, you shouldn't be desperate to get kills on him, instead you should focus in getting farm safely to be a strong damage dealer during the team fights.
Assassins are harder to play too IMO. Play a mage if you want to start with something easier. Most mages have simpler kits (not all of them) and their utility make them relevant even if they are not ahead. Assassins are mostly useless for their team when behind.
Ward a lot...
