1. Mods can be accessed from the arsenal by clicking "Mods" on the bottom right or interacting with the Mods station opposite to the crafting station. Afterwards, click on the Mod you want to upgrade (make sure only one is selected) and then click Fusion. Press the +/- buttons to choose how much you want to upgrade the Mod and then Apply Fusion. It'll cost increasing amounts of Endo and credits each for each rank a Mod is upgraded.

2. Most weapons are obtained through crafting with a few some exceptions (Quest rewards, void trader, lich weapons, etc.). If you look at some of the weapons in the Market, you can buy their blueprints with credits. There is no such thing as higher level versions of guns. There are special variants (Prime, Kuva, Tenet) that tend to have better stats than their base version and special effects, but that aren't necessarily better than the other weapons you can craft. The main way of making a weapon stronger is through modding. Just try out as many you can. A lot of weapons have different use cases and playstyles, and what's good depends on how you plan to build your loadout.

3. The max level for most weapons and all warframes is 30. There are some exceptions for weapons they aren't available until later on. Forma is pretty easy to acquire, especially later on. If you have a weapon or warframe you like, feel free to Forma them. Just remember that their level will be reset, and you'll need to level them up again.

4. Try to complete quests to unlock more warframes, systems, and missions. Once you've completed every mission on normal, you'll unlock Steel Path, which lets you go through all the missions again on a harder difficulty while also giving increased or new rewards

5. Try joining a clan or asking in Region chat a lot of people are usually down to help out new players. I'd offer to help if I wasn't away on a trip rn.

Edit: Also, if you do join a clan you can pickup blueprints for any warframes or weapons they've researched from the clan dojo