Really basic questions for ME2

Mass Effect 2 doesn't have an inventory. There are only a few weapons which you find around the game, then you can choose them at the start of each mission for each of your squad members.

Upgrades are handled very differently also. When you buy an upgrade for a weapon, you can research the data in the Normandy lab and spend the resources you collected from the planet scanning. Instead of choosing which upgrades to choose individually, all the upgrades you get stack on top of each other.
e.g. if you get an Assault Rifle AP (armour piercing) upgrade, it will automatically go onto all assault rifles. If you then get an Assault Rifle Damage upgrade, that will also go onto all your assault rifles too, and you don't have to sacrifice one for another. This happens with all weapon types.

For your second question, I am not entirely sure what you mean. However, this is a basic explanation of how levelling works in ME2:

After every mission, you will be given a set amount of XP, instead of the system from ME1 where each enemy you killed gave you a set XP amount. For most of the main missions, you will be likely to gain one level each mission.
Next time you enter a new mission, you will be shown the squad screen, where you can assign the points 9also, like ME1, in the pause menu. There is also a button to auto-level up (this is shown when each power is highlighted), which assigns points randomly to each skill tree, although you will probably prefer to do it manually to focus on what sort of build you will like for you and your squad.

Sorry if the explanations were hard to follow or sounded patronising. It might be a good idea to check the wiki: masseffect.wikia.com/wiki/Equipment#Mass_Effect_2

masseffect.wikia.com/wiki/Upgrade_Guide
masseffect.wikia.com/wiki/Powers